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Torvalds Has Harsh Words For FreeBSD Devs 571

Posted by Zonk
from the end-of-the-week-frustrations dept.
An anonymous reader writes "In a relatively technical discussion about the merits of Copy On Write (COW) versus a very new Linux kernel system call named vmsplice(), Linux creator Linus Torvalds had some harsh words for Mach and FreeBSD developers that utilize COW: 'I claim that Mach people (and apparently FreeBSD) are incompetent idiots. Playing games with VM is bad. memory copies are _also_ bad, but quite frankly, memory copies often have _less_ downside than VM games, and bigger caches will only continue to drive that point home.' The discussion goes on to explain how the new vmsplice() avoids this extra overhead."
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Torvalds Has Harsh Words For FreeBSD Devs

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  • Wrong Side of Bed? (Score:5, Insightful)

    by AKAImBatman (238306) * <akaimbatman&gmail,com> on Friday April 21, 2006 @12:05PM (#15174748) Homepage Journal
    Ok, let me see if I've got this straight:

    • Copy on Write saves you real memory, cache memory, and CPU time by pretending that each forked process has a true copy of a memory segment when it in fact is looking at the original. That is, right up until a fork tries to write to that memory location, in which case an exception is handled by making an actual copy to a new location and allowing the write.
    • Linus believes that the exception will occur enough in real world usage that it will be slower than just doing the copy in the first place.
    • Linus wants to push the manual use of zero-copy memory sharing through the vmsplice() routine. He believes that the programmer will always know better than the system when to share memory.
    • Linus doesn't like "VM Games" despite the fact that Virtual Memory, Memory Mapped Files, Disk I/O, Write Caching, etc, etc, etc, are all already "Memory Games" and "VM Games"


    Do I have that right?

    If so, I'm not really seeing his issue. Or at least not as hard-line as he sees it. The issue of memory copy performance is a tricky one, especially since CPU cycles are not the be-all to end-all of performance. Does the exception generated really cost that much more than he believes, or is it often eclipsed by the cost of the extra memory read/writes and CPU waits that are normally generated by a copy? Is it really feasible to expect program developers to do manual memory management in a day in age when programs easily weigh in at hundreds of megs?

    I'm just not sure that Torvalds is really looking at all sides of this. He may be right, but I'd like to hear more discussion between the *BSD guys and Torvalds before we put this matter to rest. And preferrably without the insults this time. :-/

    Links:

    Copy on Write as explained by Wikipedia [wikipedia.org]
    FreeBSD page on Zero Copy Patches [freebsd.org]
    Duke Uni Research [duke.edu]
    • by StarKruzr (74642) on Friday April 21, 2006 @12:18PM (#15174879) Journal
      I think Linus has gotten to the point where he just really enjoys trolling. Like, this was OBVIOUSLY uncalled-for, and he's usually such a laid-back guy. Maybe's he's read too much Slashdot. I don't know.
      • by nuzak (959558) on Friday April 21, 2006 @01:07PM (#15175409) Journal
        Actually he's been into boorish behavior from day 1 when it comes to microkernels. Namecalling between him and Tanenbaum [fluidsignal.com] (admittedly Tanenbaum is a bit haughty and provoking), and his slanderous accusations against microkernel researchers in general (a quote I can't find at the moment, but he basically accuses them all, as one big class, of academic fraud to procure grant money).

        The only microkernel Linus knows jack about is Mach, an ancient piece of crap, which indeed is Linus indeed calls it. It's unfortunate real-world systems were saddled with it, and it's got real performance issues, but Linus carries on about it like Mach ran over his dog or something.

        He conveniently ignores or chooses to remain ignorant of the fact that L4Linux is typically faster than Linux itself. To say nothing of the real-world success of QNX. And even L4Linux is pretty old by today's standards.

        This is all pretty typical behavior of Linus: bluster now, bone up and learn, and implement it later. He did so with SMP (saying famously that the way to do it was one Big F**ing Lock, then learning that no this wasn't such a great idea after all). Then he went on a tirade about sun's /dev/poll before learning that yes they actually didn't cheat and they did it smarter, and Linux followed.

        Ultimately, Linus and Linux come around. Sometimes he just has to vent.
        • what the? L4Linux has to run on top another REAL kernel, usually Linux. QNX is a realtime operating system, not a general purpose desktop or server one. And where's the spicy name-calling, I see the minix way being called "brain-damaged", heheh, but maybe I missed a good personal zinger? Mach probably bugs Linus because its still being used, even for relatively (in comparison to Linux) new projects & newer commercially sold OS.
        • What harsh words? (Score:5, Insightful)

          by Inoshiro (71693) on Friday April 21, 2006 @03:34PM (#15176834) Homepage
          Andy went out and said that he thought the Linux approach was wrong, and archaic, and that people should go and wait for GNU.

          Linus said that he felt this was wrong, and that being a prof is no excuse for Minix being the mess it was (and Minix was a mess in the late 1980s/early 1990s). He also apologized if he came off as too harsh for his writing about how people should be able to throw away an old design in favour of a new one anyway, etc.

          It was very polite compared to some of the non-Andy/Linux replies.
      • by arivanov (12034) on Friday April 21, 2006 @02:45PM (#15176370) Homepage

        More likely he had some really bad acid the previous night.

        After all he did more than 6 revisions of the Linux VM using CopyOnWrite before this latest fad.

        Possibly more.

        Off the top of my head that is at least 1 in the 1.2 tree, 1 in the 2.0 tree, 1 in the 2.2 tree, 2 in the 2.4 tree and more than 2 in the 2.6 tree, all of which being CopyOnWrite and at least some of which has been hailed as the next best thing after hot bread.

        As far as the technical point he is possibly correct for x86 where COW goes through the fault mechanism and causes some TLB and cache abuse which is really bad on modern CPUs. I am not sure as far as other architectures are concerned, because IIRC (I may be wrong) the memory mapper hardware on the old Sparc was designed for COW in first place.

        Anyway, before calling somebody else an idiot for something you have happily done for 10+ years till yesterday it may be nice if you look at yourself in the mirror. Because I never remember any branch of FreeBSD reaching the point where you can do a find /usr -exec cat {} > /dev/null \; to hang the system. That is 2.6.16 at your service (from rc4 onward) on at least two x86 subarchitectures where I had the time to test it. That is besides the unkillable processes in [S] state on an nfs flock in 2.6.14 (yep, that is a gem which no other unix has managed so far), besides the OOM idiocies in 2.6.10, besides deliberately making it absolutely impossible to backtrack any more interesting patch to a previous kernel without employing a team of kernel developers because the VM and locking is not compatible across any kernel version since 2.6.9 and even when it is something else is changed like the tty layer, besides.... Aarghh.....

      • by bfields (66644) on Friday April 21, 2006 @03:18PM (#15176691) Homepage
        I think Linus has gotten to the point where he just really enjoys trolling.
        Could be [theaimsgroup.com]:
        I got slashdotted! Yay!

        On Thu, 20 Apr 2006, Linus Torvalds wrote:
        >
        > I claim that Mach people (and apparently FreeBSD) are incompetent idiots.

        I also claim that Slashdot people usually are smelly and eat their
        boogers, and have an IQ slightly lower than my daughters pet hamster
        (that's "hamster" without a "p", btw, for any slashdot posters out
        there. Try to follow me, ok?).

        Furthermore, I claim that anybody that hasn't noticed by now that I'm an
        opinionated bastard, and that "impolite" is my middle name, is lacking a
        few clues.

        Finally, it's clear that I'm not only the smartest person around, I'm also
        incredibly good-looking, and that my infallible charm is also second only
        to my becoming modesty.

        So there. Just to clarify.

        Linus "bow down before me, you scum" Torvalds
    • by qwijibo (101731) on Friday April 21, 2006 @12:18PM (#15174880)
      I don't consider myself an expert in kernel programming, but I definitely think someone is off base if they're expecting programmers as a whole to do the right thing. Many programs seem to work by coincidence rather than design. People didn't do their memory management right in the days when it was necessary. Now that a lot of people are moving towards languages that handle the memory management for them, I expect even fewer to worry about it. That does mean that the programmers of the programming languages are the ones who are responsible, but I'd personally rather have the kernel take a more active role in memory management.
      • by LordNimon (85072) on Friday April 21, 2006 @01:22PM (#15175581)
        I don't consider myself an expert in kernel programming, but I definitely think someone is off base if they're expecting programmers as a whole to do the right thing.

        Well, I am an expert in kernel programming, and I can tell you that Linus has little tolerance for anyone who doesn't program the way he does. That's one reason, for example, that he doesn't support debuggers. Every other OS has a kernel debugger built-in (and therefore, generally stable and full-featured), but not Linux. Even the OS/2 kernel debugger that was created 10 years ago is better than anything Linux has.

    • I think the point is should the programmer be forced to allways have COW enabled or be able to choose. It seems to make sence that the programmers could do some application profiling to figure this out and what bits it makes sence for. As to what should be the default that realy depends on your workload and is debatable I would hope it gets added to proc so it can be altered by process and global default at runtime.
    • by jandrese (485) <kensama@vt.edu> on Friday April 21, 2006 @12:20PM (#15174895) Homepage Journal
      One thing that concerns me about making all of these copies is that it seems like a quick and easy way to blow out your L2 cache. That could in the long run have a worse performance penalty than having to play the VM tricks with CoW.
      • by mrsbrisby (60242) on Friday April 21, 2006 @12:26PM (#15174960) Homepage
        One thing that concerns me about making all of these copies is that it seems like a quick and easy way to blow out your L2 cache. That could in the long run have a worse performance penalty than having to play the VM tricks with CoW.

        No it won't. The only way to avoid the copies is to avoid the pagefaults. Since userland doesn't get explicit notification in FreeBSD of when the pages are safe to use, the process should wait as long as possible (e.g. until malloc() starts failing)

        The idea Linus pushes here is explicit notification- via select() or poll() and returnable via recvmsg(). That way the userland knows exactly which pages can be reused.

        The result is that it's faster and easier to develop userland programs to take advantage of it. It's also easier to degrade gracefully into read()/write() until the FreeBSD people see the light and add support for this too.

        It's really a clever idea.
      • by hackstraw (262471) * on Friday April 21, 2006 @12:47PM (#15175154)
        One thing that concerns me about making all of these copies is that it seems like a quick and easy way to blow out your L2 cache. That could in the long run have a worse performance penalty than having to play the VM tricks with CoW.

        Right. Especially with multithreaded apps as Linus pointed out. Also the TLB misses could get expensive as well, and again the TLB misses will be more of an overhead with multithreaded apps.

        I don't believe that COW is completely evil. It exists, obviously for a reason, but I would agree with Linus on a much less harsh tone (depending on mood).

        Oh, and isn't "VM" a trick to begin with?

    • by mrsbrisby (60242) on Friday April 21, 2006 @12:22PM (#15174918) Homepage
      Copy on Write saves you real memory, cache memory, and CPU time by pretending that each forked process has a true copy of a memory segment when it in fact is looking at the original. That is, right up until a fork tries to write to that memory location, in which case an exception is handled by making an actual copy to a new location and allowing the write.

      No. Updating the page tables twice and having a fault in there is very expensive.

      Linus believes that the exception will occur enough in real world usage that it will be slower than just doing the copy in the first place.

      And he's right too. But he's not recommending the copy "in the first place" - he's recommending explicit notification that the pages aren't used anymore instead of an implicit notification by-way of a page fault.

      Linus wants to push the manual use of zero-copy memory sharing through the vmsplice() routine. He believes that the programmer will always know better than the system when to share memory.

      That's correct.

      Does the exception generated really cost that much more

      Yes. There isn't a grey area on it either- it's basic math: cost of page copy + exception + 2 * (page table update) is greater than cost of page copy + page table update.

      The real issue is that the userland knows what it's doing. Eventually it'll want to reuse a buffer. Now does the userland start reusing pages when malloc() fails- thus incuring the exceptions when memory is tight? Or does it reuse them when the kernel says they're reusable?

      The latter makes more sense if you're actually concerned about performance. The former may be easier to code, but I doubt many people will actually do that because it's hard to test.

      In practice what people do is use a static buffer- that's even EASIER to code, but it means page faults happen ALL the time.

      Is it really feasible to expect program developers to do manual memory management in a day in age when programs easily weigh in at hundreds of megs?

      They already have to do it. Whether it's the BSD implementation or the new Linux implementation they already have to do it if they want reasonable performance in the real world.

      To really take advantage of the BSD implementation, your program needs to monitor malloc() usage, and start attempting to reuse pages when it fails- oldest to newest. This is complicated and hard to test.

      To really take advantage of the Linux implementation, your program waits until it gets notification (via select() or poll()) on the vmsplice() recvmsg() operation. Once that occurs, the notification says exactly which pages can be used.

      The result? Userland on Linux is easier to write, and easier to test. It'll also be faster.
      • I'm not an expert on any of this, but what I do know is that when you start using up a lot of memory Linux totally sucks. On a 256 MB RAM machine, with about twice that amount of swap, if I run over 50% memory usage the system becomes unusable for long periods of time. Even at much greater loads, FreeBSD just feels slightly sluggish at worst. This has been true for years. It was the main reason many people I know refused to use linux (they went for either commercial Unix or the BSDs). It's still true w
      • Yes. There isn't a grey area on it either- it's basic math: cost of page copy + exception + 2 * (page table update) is greater than cost of page copy + page table update.

        There you're assuming that the page copy will be necessary. In cases where the W in COW does NOT occur, isn't COW much better?

        (Please don't mod me up. I have no idea what I'm talking about.)
        • by mrsbrisby (60242)
          There you're assuming that the page copy will be necessary. In cases where the W in COW does NOT occur, isn't COW much better?

          Well, no, it's about the same actually.

          The problem is that in the naive implementation, the page copy is always necessary. A complicated implementation (in userspace) to take advantage of COW is more complicated than with explicit notification.
      • by AKAImBatman (238306) * <akaimbatman&gmail,com> on Friday April 21, 2006 @12:38PM (#15175071) Homepage Journal
        I *think* I understand what you're saying. Basically, the problem is caused by the fact that usermode code never (or rarely, depending on your platform) releases any of the memory it has allocated. Instead, it keeps reusing the same memory pools over and over again. This becomes a problem with CoW because the kernel doesn't learn about the deallocation of memory until the usermode reallocates it for another purpose. When that reallocation happens, the read-only exception is going to be triggered. Thus there's going to be a 100% occurance of exceptions on CoW pages.

        However, given that the "free()" routine is part of the OS in FreeBSD, wouldn't it make sense to create a smarter "free()" routine that would attempt to recognize and explicitly deallocate CoW pages?
        • by mrsbrisby (60242)
          Basically, the problem is caused by the fact that usermode code never releases any of the memory it has allocated.

          Oh no. That's the solution actually. :)

          The problem is in using a static buffer instead of allocating a buffer for each send operation. If you use a static buffer, you ALWAYS cause a fault. If you malloc() each time, you won't fault- at least until you reuse the pages later (when malloc() fails).

          However, given that the "free()" routine is part of the OS in FreeBSD

          No, it's not unfortunately. It's
          • by AKAImBatman (238306) * <akaimbatman&gmail,com> on Friday April 21, 2006 @01:01PM (#15175352) Homepage Journal
            If you use a static buffer, you ALWAYS cause a fault.

            * Lightbulb goes on

            Oohhhh, I see! So something like this is the problem:
            char buffer[1024];
            int read = 0;
            int length;
             
            while(read < totalSize)
            {
                length = fread(buffer, 1, 1024, &file);
                read += length;
             
            //Do some stuff, but don't free the buffer!
            }
            What you're saying is that every time through the loop, there's going to be a page fault as the CoW pages are wiped away by the new copy into the same logical buffer. CoW is dependent on allocating new pages every time so that you don't ever write to the old CoW pages. Correct?

            Of course, this is where I'd really like to hear from the *BSD developers. Surely they must be aware of this issue? Do they expect programmers to throw away their buffers, or do they have a plan?
            • by mrsbrisby (60242) on Friday April 21, 2006 @01:25PM (#15175619) Homepage
              What you're saying is that every time through the loop, there's going to be a page fault as the CoW pages are wiped away by the new copy into the same logical buffer. CoW is dependent on allocating new pages every time so that you don't ever write to the old CoW pages. Correct?

              Exactly correct. Those frequent CoW operations are slow- the page faults are expensive. If you had instead written:

                      char *buffer;
                      int read = 0;
                      int length;

                      while(read < totalSize)
                      {
                              buffer = malloc(1024);
                              length = fread(buffer, 1, 1024, &file);
                              read += length; //Do some stuff, but don't free the buffer!
                      }


              Then it would operate quickly on FreeBSD. The problem then becomes exactly when do you free all those malloc()s?

              On Linux, you can get a signal from the kernel- via a recvmsg() call that will tell you exactly which pages are now available to be freed- or better still, reused.

              It'll be easy to check and test correctness AND the programmer has to be aware it's going on in order to use it at all.

              Under FreeBSD the programmer can use the syscall, but never get the performance unless they know exactly what's going on.

              Of course, this is where I'd really like to hear from the *BSD developers. Surely they must be aware of this issue?

              I don't know. The article wasn't about that- I doubt Linus pays attention to what the BSD people know- in fact, I don't even think he knows for certain if FreeBSD even works this way. :)

              The point is that using CoW is stupid for this. It makes things complicated in the hard case, and in the easy case, it makes things slower.
              • by Sangui5 (12317) on Friday April 21, 2006 @02:12PM (#15176101)

                Then it would operate quickly on FreeBSD. The problem then becomes exactly when do you free all those malloc()s?

                No, it'd be slower than just copying on FreeBSD too.

                while(read < totalSize){
                buffer = malloc(1024); //1024 is < pagesize!
                length = fread(buffer, 1, 1024, &file);
                read += length; //Do some stuff, but don't free the buffer!
                }

                This is where VM games really bite you in the ass, because you get false sharing. Even if you never reuse the buffer, this can cause 3 copies--each group of 4 (3.99ish) buffers will be on the same page, and therefore each call will cause a fault from the previous one.

                In theory the OS could be allow itself write & check for overlapping calls (& avoid the COW fault), but note that the read() example really isn't interesting for zero-copy unless you're using hardware TCP offloading. Zero copy is more interesting for write(). The usual case is then:

                while(){
                b = malloc();
                fill_in_buffer(b);
                write(b);
                }

                and that fill_in_buffer step *must* cause a fault if sets of buffers are on the same page. To avoid COW faults you have to be really careful that you don't accidentally write to the same page as the buffer--even indirectly by malloc updating it's inline data structures. That's pretty nasty to do--the easiest way is to allocate 8K at a time, and use a page-aligned chunk from the middle of it. Talk about a waste of memory.

      • by tearmeapart (674637) on Friday April 21, 2006 @12:50PM (#15175191) Homepage Journal
        >> Linus wants to push the manual use of zero-copy memory sharing
        >> through the vmsplice() routine. He believes that the programmer
        >> will always know better than the system when to share memory.
        >
        > That's correct.

        No, that is not always correct.

        I am a C developer for a large multinational corporation that likes to make money. When I need to fork(), I do not have the time to think of all the memory management invovled with fork(). I just want it to be done reliably, and I want it to be done fast.

        If it turns out that my code runs 10% faster on FreeBSD than on Linux, than that means that the code is probably going to go on a FreeBSD system. And if FreeBSD is not an option, than I am not going to do the optimization (because CPUs cost less than my wages).
        Also: optimization never happens anyways (or at least, not properly).

        So from my perspective:
        I want the kernel to run my code as fast as possible by default.
        • by mrsbrisby (60242)
          When I need to fork(), I do not have the time to think of all the memory management invovled with fork(). I just want it to be done reliably, and I want it to be done fast.

          So what? Who's talking about fork()?

          This is about copy-on-write of zero-copy fifos and TCP. If you don't know what the rest of us are talking about, please just say so, and we'll be happy to tell you exactly what's going on.

          Maybe you'll have something to contribute at that point, or maybe you'll just learn something.

          And if FreeBSD is not
        • by Jherek Carnelian (831679) on Friday April 21, 2006 @02:21PM (#15176182)
          When I need to fork(), I do not have the time to think of all the memory management invovled with fork().

          This has NOTHING to do with fork(). You are used to CoW (copy-on-write for anyone else reading along) only applying to fork(), but that is not the issue under discussion at all. You, and probably 95% of the responders here, need to go RTFA.

          The issue is implementing zero-copy IO. FreeBSD's way of doing it do a setsockopt() that causes any write() on that socket to mark the buffer CoW so that it can use it exclusively for handing down to the device driver. The "magic" is that if the programmer tries to use that buffer while the device driver owns it he will get a copy. BUT, the programmer has no way of knowing when that buffer is available again.

          Linus's point is that marking a page CoW is very expensive - especially in an SMP environment, almost as expensive as just copying that page to begin with would be. He also argues that taking a page-fault to invoke the CoW to a new page, or simply to turn off the CoW attribute, is orders of magnitude more expensive than just copying it in the first place.

          So that means the CoW for sockets is only really useful if you rarely or never reuse your buffers again. And the only place that happens is in synthetic benchmarks.

          If Linus had said "Microsoft is a bunch of idiots for implementing a feature that only looks good on benchmarks" everybody would be nodding their heads in agreement. I think the reason people are not doing the same here is because they just don't understand the details.

    • by QuietLagoon (813062) on Friday April 21, 2006 @12:23PM (#15174925)
      Why does Torvalds care? I mean, why does he care what FreeBSD is doing, or how they are doing it?

      If there is something that FreeBSD does that he likes, he is welcome to the code. If there is something that FreeBSD does that he does not like, he can just let it go.

      Why does he feel the need to start a war within the OpenSource community?

    • by RailGunner (554645) * on Friday April 21, 2006 @12:25PM (#15174955) Journal
      Yes, you've got it right on what Linus is saying.

      The issue of memory copy performance is a tricky one, especially since CPU cycles are not the be-all to end-all of performance. Does the exception generated really cost that much more than he believes, or is it often eclipsed by the cost of the extra memory read/writes and CPU waits that are normally generated by a copy? Is it really feasible to expect program developers to do manual memory management in a day in age when programs easily weigh in at hundreds of megs?

      What programs weigh in at hundreds of megs? Don't count data files or map files for games. The entire bin directory of a PostgreSQL install is only 20 megs, and that's a lot of stuff there.

      And as far as doing memory management... YES. I have yet to see a compiler do a better job at managing memory than what I can do when I write my code - and the reason is quite simple: I'm the domain expert, not the compiler. Compilers generally do a good job, but it's those specific cases that bite you over and over again.

      Linus is also right about child threads writing to memory. If that never happened, we wouldn't have a concept of a lock or a semaphore. The bottom line is that is happens a lot.

      He may be right, but I'd like to hear more discussion between the *BSD guys and Torvalds before we put this matter to rest. And preferrably without the insults this time. :-/

      I agree, the ad hominem was completely unecessary.

    • Copy on Write saves you real memory, cache memory, and CPU time by pretending that each forked process has a true copy of a memory segment when it in fact is looking at the original. That is, right up until a fork tries to write to that memory location, in which case an exception is handled by making an actual copy to a new location and allowing the write.

      I think the problem with this approach is that COW will only give you a copy of the particular piece of the memory that you accessed. That means that
      • by mrsbrisby (60242)
        I think the problem with this approach is that COW will only give you a copy of the particular piece of the memory that you accessed. That means that the system has to keep huge tables of what is shared and what is not and every time you make a call to request ANY memory it's going to need to check the table. This action is going to result in an overall performance degradation since the application has to check the table for every write over the long-haul, rather than just duplicate the memory and go.

        It doe
        • by dgatwood (11270)

          What Linus suggests is explicit notification- say a select() or poll() operation that says "these pages are now free". This works out well, and is indeed faster because there aren't any copies or page faults. It's also easier to develop.

          Problem is that unless you're talking about declaring the pages "free" by storing more data in the heap info structure, declaring the pages free would require trapping into the kernel, and that is every bit as slow as the exception on most architectures, only now you're

          • by mrsbrisby (60242)
            Problem is that unless you're talking about declaring the pages "free" by storing more data in the heap info structure, declaring the pages free would require trapping into the kernel, and that is every bit as slow as the exception on most architectures, only now you're doing it more often, since you're doing it every time a page changes from free to not free.

            No. System calls are not as slow as exceptions.

            If they are on your architecture, you're not supporting a million clients at a time on that architectur
    • by Peaker (72084)
      Ok, let me see if I've got this straight:

      Copy on Write saves you real memory, cache memory, and CPU time by pretending that each forked process has a true copy of a memory segment when it in fact is looking at the original. That is, right up until a fork tries to write to that memory location, in which case an exception is handled by making an actual copy to a new location and allowing the write.
      Linus believes that the exception will occur enough in real world usage that it will be slower than just doing th
  • Development-wise, how much is Debian's FreeBSD port [debian.org] from Debian versus FreeBSD? Or are its advancements in tandem with both projects. And does either half suffer from the combination of the other?
  • by digitaldc (879047) * on Friday April 21, 2006 @12:06PM (#15174760)
    Playing games with VM is bad.

    I know, I hate it when I have to listen to 26 hang up messages in my inbox only to find out someone is playing games with me. :(
    • by OctoberSky (888619) on Friday April 21, 2006 @12:15PM (#15174846)
      As a Slashdot user there is no way in hell you have 26 messages on your phone machine. Maybe 3 messages, but thier probably your mom calling to ask you when your coming out of the basement, your friend inviting you to stand in line for tickets to the latest Sci-Fi flick and the Pizza guy confirming your order of 2 large and a 2 liter of Mt. Dew on a Friday night.
      • Damn now I want pizza!
      • You missed the key factor -they are hang-up calls from someone playing a joke.

        He didn't say they were legitimate messages from real people who actually wanted to talk to him.
      • thier probably your mom calling to ask you when your coming out of the basement, your friend inviting you to stand in line for tickets to the latest Sci-Fi flick and the Pizza guy confirming your order of 2 large and a 2 liter of Mt. Dew on a Friday night

        You were almost right, but it was my abusive step-dad to tell me to come out of my attic room and clean up his beer cans, my online friend from Japan inviting me to come to Tokyo for a Manga festival, and the Thai delivery service confirming my order of
  • Tantrums (Score:4, Funny)

    by ClamIAm (926466) on Friday April 21, 2006 @12:06PM (#15174762)
    Methinks we need to start tagging "tantrum" to this type of thing.
  • Did you hear that? (Score:2, Insightful)

    by DaHat (247651)
    It sounded like the opening volley of the second great Unix war, only this time instead of pitting proprietary Unix vendors and systems against each other... it is two open source ones.

    It will be interesting to see what weapon the BSD crowd will retaliate with.
    • It will be interesting to see what weapon the BSD crowd will retaliate with.

      I would just prefer that their response is to release a stable system using their method.
    • by arosas (904929)
      With a comment like that, I can only imagine the kind of temper tantrum that Theo de Raat will throw. I mean honestly, whatever happened to common courtesy? There's no need for such comments like "incompetent idiots". I see so many people push for the advancement of OSS, only to find that it was in vain thanks to school-yard hissy fits like this.
    • It will be interesting to see what weapon the BSD crowd will retaliate with.

      My guesses are they will respond something like this:

      FreeBSD: FreeBSD users will continue their campaign of random acts of elitist snobbery against Linux users.
      OpenBSD: Theo will threaten to stop work on OpenSSH unless Linus gives him $10,000 for every nasty email he sends a *BSD developer.
      NetBSD: Will stop developing the NetBSD port for Linus' microwave.
    • Obviously a drive-by shouting match.

      "Oh Reginold! I disagree!!!"
  • by Anonymous Coward on Friday April 21, 2006 @12:12PM (#15174819)
    kernels, me thinks it's just sour grapes because Linus can't compete in that area.
  • by Anonymous Coward
    Is that my COW?
    it goes "incompetent idiots."
    It is a Torvalds.
    That is not my COW.
  • East Coast kernel-fu vs. West Coast kernel-fu.... FIGHT!
  • This is so tech I don't even undersatnd what they are talking about yet I am very "Intellectually Curious" [slashdot.org].
  • by Animats (122034) on Friday April 21, 2006 @12:22PM (#15174917) Homepage
    This is an old idea, and it's been tried before. I think it was first tried by Jerry Popek at UCLA in the 1980s, and it was tried in Mach.

    The basic idea is to fake some memory to memory copying operations by using the virtual memory hardware. More specificially, the idea is that when you do a big "write", the space just written becomes read-only to the writing process, rather than being actually copied. When the write is complete, read-only mode is turned off. This eliminates one copy.

    The trouble with this is that when you manipulate the page table to do that, you have to do some cache invalidation. That usually results in cache misses, which outweigh the cost of the copy. So this usually is a lose. Linus points out that it looks good on benchmarks, because benchmarks typically aren't using data for anything and thus don't experience the cache misses.

    Actually, copying is a relatively cheap operation in modern CPUs unless the copy is huge, since most of the work is done in the caches. The mania for "zero copy" complicates systems considerably, makes them less reliable, and, in the end, usually doesn't speed up real work by much.

    Some of this mania comes from Microsoft FUD. At one time, Microsoft was claiming that an "enterprise OS" must be able to serve web pages from inside the kernel. This led to more Linux interest in "zero copy" approaches to be "competitive".

    • I don't know about the rest of your post, but your explanation of CoW is confusing and inaccurate:

      The basic idea is to fake some memory to memory copying operations by using the virtual memory hardware. More specificially, the idea is that when you do a big "write", the space just written becomes read-only to the writing process, rather than being actually copied. When the write is complete, read-only mode is turned off. This eliminates one copy.

      The way CoW works is that when a process copies something

      • n8_f is correct that the parent post is full of bunk. In addition to what n8_f said, the cost isn't cache coherency, it's the additional copy you end up doing -anyway- if the data does get modified (which is likely when CoW is used for an I/O buffer) on top of messing with the page tables. Messing with the page tables is especially expensive when you need to syncronize this across multiple processors. Given that most new systems have multiple cores, CoW loses even more benefit in any case where the CoW even
    • Torvalds:

      I claim that Mach people (and apparently FreeBSD) are incompetent idiots. Playing games with VM is bad. memory copies are _also_ bad, but quite frankly, memory copies often have _less_ downside than VM games,

      I totally agree with that, I just go one step further and say that Torvalds is also a total idiot: VM games are bad, so use copying instead because that's less bad. But copying is also bad, so why it at all? Neither are good solutions.

      The problem is that linux and bsd are using "virtual memor

  • by Anonymous Coward on Friday April 21, 2006 @12:24PM (#15174943)
    .. this [wikipedia.org] will help you keep yourself calm.
  • the oracle has spoken and we are struck with awe...

    linus is just a guy, quite opinionated, and quite harsh in his words at times. are we gonna see this kind of slashdot story everytime he misbehaves somewhat? (i hope not)
  • ....more rivalry between the BSD folks and the Linux folks. Using phrases like "incompetent idiots" lifts this out of 'friendly sibling rivalry' towards Holy War territory.

    I dig Linus, he's a smart, capable and funny guy, but this kinda dissapointed me. He's better than that.

  • Sweet (Score:5, Funny)

    by bogie (31020) on Friday April 21, 2006 @12:30PM (#15175001) Journal
    It's been a while since we had a huge linux vs BSD flame feast.

    I'll start.

    BSD user: Linux is a confusing mess of programs and is less stable than BSD.

    Linux user: Your still here? I thought you were dead by now?
  • Who pissed in Linus' Wheaties?

    Or did someone dress his plush tux up with an "I love Windows" sign and BSD daemon horns?

  • by cpu_fusion (705735) on Friday April 21, 2006 @12:37PM (#15175064)
    And in other news...
    Grass is green;
    Oil is overpriced;
    Absolute power corrupts absolutely.

  • by Electric Eye (5518) on Friday April 21, 2006 @12:40PM (#15175093)
    Our top story tonight, uber geek Linus Torvalds unleashed a scathing indictment of some other geeks, claiming they are skating on thin ice by using Virtual Memory calls to improve performance. The words sparked outrage in the dark rooms of colleeg geek programmers from Berkley to Berlin. The angry geek mobs said they're going to launch a flame war from their computers "to teach Linus a lesson."

    In the words of George Takei "Hoooooooly geeeez!" This is news??
  • by lymond01 (314120) on Friday April 21, 2006 @12:43PM (#15175120)
    "I claim that Mach people (and apparently FreeBSD) are incompetent idiots."

    Linus, who's becoming more outspoken as he ages, needs to find that line between anonymous forum geek and software spokesperson...and then not cross it. Calling anyone an incompetent idiot is both non-constructive if you're hoping to improve a situation, and just plain unfriendly in an area where cooperation amongst developers is so crucial (open source).
  • by Admiral Burrito (11807) on Friday April 21, 2006 @12:46PM (#15175143)
    Linus Torvalds: "Don't have a COW, man!"
  • by Shohat (959481) on Friday April 21, 2006 @12:59PM (#15175323) Homepage
    Linus is a gifted engineer ,let him be rude . Aside from Linus being rude , there is no actual story here .
    I used to own restourant and also an Office supplies shop . It was quite interesting and made me some money , but I hated the fact that the most important factor in my life was pleasing(customers) or fighting(suppliers) other people . I had to constantly think what to say and how to behave .
    I am no longer a business owner , and now I work with a rather gifted bunch of engineers , and frankly it gives me great pleasure to know that neither I nor the people I work with dont really care about being polite , clean shaven well spoken or good looking . I can be rude if I want to , they can be rude if they want to , and we all get along very well .
  • Lesson on Tact.. (Score:3, Insightful)

    by corellon13 (922091) on Friday April 21, 2006 @01:06PM (#15175406)
    Linus, you may be right and you may be very smart, but you should try a little tact. Here's a good definition for it that I learned from a drill sergeant: "Tact is the ability to tell someone to go to hell and look forward to the trip."

    Being nice and respectful doesn't mean you can't tell it like it is.
  • by Gothmolly (148874) on Friday April 21, 2006 @01:35PM (#15175730)
    Linus has frequently called people idiots, and ignored patches, and done stuff his own way for a very long time now. He's quite successful at it. Perhaps what most people need to realize is that he is that good, that he can. The average read-Slashdot-during-work-while-coding Slashdotter is not in his league, so decrying his adhominem attacks, or "I would do X instead" arguments just dont hold much water.
  • by pammon (831694) on Friday April 21, 2006 @02:08PM (#15176062)
    Can someone please explain to me how this new proposal is different from the aio_* functions (asynchronous I/O) that appeared in FreeBSD?

    For example, aio_write() writes to the file descriptor, allows you to poll for success a la select, and tells you not to modify the buffer before it's done (but doesn't try to stop you with copy-on-write).

    This sounds exactly like what Linus wants.

  • An explanation (Score:5, Informative)

    by sjames (1099) on Friday April 21, 2006 @04:17PM (#15177223) Homepage

    There seem to be a LOT of misconceptions about the discussion of vmslice() vs COW vs copy. This has nothing to do with conserving memory and everything to do with high performance I/O. If your app just needs to send a couple small files from A to B, you probably don't care about this at all.

    A little background is needed on the terminology and mechanisms of I/O for any of this to make sense. For an example, let's say your app is a very busy web server sending dynamic (but trivial to compute) pages out.

    The oldest and simplest method is copy. The app calls write(int sock, char *buffer, int length) on a socket. The kernel coppies the contents of buffer from userspace memory into a kernel space buffer and at least queues the data to the TCP stack before returning.

    COW is an attempt to avoid the cost of copying the outgoing data.. In that case, the reference count on the physical pages that make up buffer is bumped up (since now kernel and application are both interested in them), and marks the pages as COW. That is, the virtual memory addresses are set as read only and a flag bit is set (more or less). The latter is done so the kernel needn't worry about them again. By the time the write call returns, the app is able to immediatly write to that memory (sorta) without worry.

    When that write happens, the app takes a page fault (writing to a read-only page). The kernel sees that the pages are COW, copies the data to a new physical page, and maps the page in read/write. Then it returns from the fault. OTOH, if the kernel finished with the page first (the data goes on to the wire), it re-marks the page(s) so the app can access them without a copy.

    The hope is that often enough, the app WON'T try to write to the pages while they're busy and so the cost of that copy is saved. If that hope comes through often enough it MIGHT be vaguely uesful. I say MIGHT since there is a significant cost just for marking the pages (the CPU's TLB must be flushed for the change to take effect). If the faults happen, it's a BIG loss since handling a fault takes thousands of CPU cycles.

    So, for it to have any chance to help, the application programmer must already know enough to TRY to avoid writing to the same buffer again until it gets to the wire. Unfortunatly, it can never be sure so most apps don't bother.

    The vmsplice() proposal is fairly simple. In this case, the app explicitly requests special treatment of the write. The pages are NOT marked as read only at all. Instead, the app is on it's honor to leave them alone until the kernel notifies it that they are again available. This saves the copy and the costs of TLB flush AND the (potential) cost of page faults. If the app breaks it's promise, it is the only one to suffer as the data it sent is corrupted (no kernel housekeeping is ever stored in such pages so there are no security implications). Any damage the app might do by sending screwy data could also be done using the old copy method.

    What it all comes down to is that playing tricks with page mapping LOOKS nice at first glance since it SEEMS reasonable that not copying bytes around will save CPU cycles and memory bandwidth. The re-mapping (or just permission changes) on pages SEEMS lightweight. Unfortunatly, in fact, re-mapping or changing permission forces cache invalidations and page faults are just plain expensive. With the direction CPU design is going, these things will likely get more expensive rather than less (as they have for most of the history of microprocessor design).

    It's really not that complex for an application to use. At least in comparison to the complexities and level of knowledge required to write an app that performs well enough to need this in the first place.

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