+chandlerc, aaronballman, in case there are additional carryovers and/or issues from the review thread which I've left out. I have a patch up for review[1, 2] that attempts to replace LLVM's mutex implementation with std::mutex and std::recursive_mutex. While the patch seems to work, there are questions surrounding whether or not the approach used is correct. I'll try to summarize the issues as best I can, in hopes of getting some feedback from a broader audience, to make sure this is done correctly: 1) Should support multi-threading be a compile-time or runtime parameter in LLVM? Currently it is both. It is compile-time through the use of the define LLVM_ENABLE_THREADS, and it is runtime through the use of functions llvm_start_multithreaded, llvm_is_multithreaded, etc. I and some others feel like runtime support for multi-threading could be removed, and it should be compile-time only. However, I am not aware of all the ways in which this is being used, so this is where I would like some feedback. The issues I have with runtime multithreading support are the following: * It leads to confusing code. At any given point, is multi-threading enabled or disabled? You never know without calling llvm_is_multithreaded, but even calling that is inherently racy, because someone else could disable it after it returns. * It leads to subtle bugs. clang_createIndex, the first time it's called, enables multi-threading. What happens if someone else disables it later? Things like this shouldn't even be possible. * Not all platforms even support threading to begin with. This works now because llvm_start_multithreaded(), if the compile time flag is set to disable threads, simply does nothing. But this decision should be made by someone else. Nobody really checks the return value from llvm_start_multithreaded anyway, so there's already probably bugs where someone tries to start multi-threading, and it fails. * What does it actually mean to turn multi-threading support on and off? Anybody that tries to do this is almost certainly broken due to some edge cases about when it's on and when it's off. So this goes back to the first two points about confusing code and subtle bugs. 2) What should happen when you try to acquire a mutex in an app with threading disabled? If this is a compile-time parameter, the solution is simple: make an empty mutex class that satisfies the Lockable concept, and have its methods do nothing. Then typedef something like llvm::mutex to be either std::mutex or llvm::null_mutex accordingly. If this is a runtime parameter, it's more complicated, and we should ask ourselves whether or not it's necessary to avoid the overhead of acquiring an uncontended mutex in single threaded apps. For what it's worth, all reasonable STL implementations will use a lightweight mutex implementation, which generally require less than 100 nanoseconds to acquire uncontended, so I think we don't need to care, but again some feedback would be nice. 3) We have some debug code in our mutex implementation that is intended to try to help catch deadlocks and/or race conditions. Do we need this code? I think we can get by without it, but again some feedback about how, if at all, people are using this would be nice. For example, if you want to detect deadlocks and race conditions you can use TSan. Thanks! Zach [1] - http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140602/220051.html [2] - http://reviews.llvm.org/D4033 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140606/568850ca/attachment.html>
Zach, thanks for bringing the discussion to the full mailing list. While some of this was hashed out in the review thread, I'll drop my personal feelings here. Maybe will encourage others to chime in... On Fri, Jun 6, 2014 at 6:47 PM, Zachary Turner <zturner at google.com> wrote:> 1) Should support multi-threading be a compile-time or runtime parameter > in LLVM? > > Currently it is both. It is compile-time through the use of the > define LLVM_ENABLE_THREADS, and it is runtime through the use of functions > llvm_start_multithreaded, llvm_is_multithreaded, etc. I and some others > feel like runtime support for multi-threading could be removed, and it > should be compile-time only. However, I am not aware of all the ways in > which this is being used, so this is where I would like some feedback. The > issues I have with runtime multithreading support are the following: > > * It leads to confusing code. At any given point, is multi-threading > enabled or disabled? You never know without calling llvm_is_multithreaded, > but even calling that is inherently racy, because someone else could > disable it after it returns. > > * It leads to subtle bugs. clang_createIndex, the first time it's called, > enables multi-threading. What happens if someone else disables it later? > Things like this shouldn't even be possible. > > * Not all platforms even support threading to begin with. This works now > because llvm_start_multithreaded(), if the compile time flag is set to > disable threads, simply does nothing. But this decision should be made by > someone else. Nobody really checks the return value from > llvm_start_multithreaded anyway, so there's already probably bugs where > someone tries to start multi-threading, and it fails. > > * What does it actually mean to turn multi-threading support on and off? > > Anybody that tries to do this is almost certainly broken due to some edge > cases about when it's on and when it's off. So this goes back to the first > two points about confusing code and subtle bugs. >I share your concerns here. I think that it is a complete mistake to support this at runtime. Note, I'm not saying that we should penalize single threaded users of LLVM. There are many legitimate uses of LLVM which reserve exactly one thread for it and it seems reasonable for LLVM to try to make sure that use case continues to work. I don't think removing the runtime configuration of this would impact those users in any measurable way, or if it does I think those are all bugs in how LLVM works and should be fixed irrespective of this decision.> > 2) What should happen when you try to acquire a mutex in an app with > threading disabled? > > If this is a compile-time parameter, the solution is simple: make an empty > mutex class that satisfies the Lockable concept, and have its methods do > nothing. Then typedef something like llvm::mutex to be either std::mutex > or llvm::null_mutex accordingly. If this is a runtime parameter, it's more > complicated, and we should ask ourselves whether or not it's necessary to > avoid the overhead of acquiring an uncontended mutex in single threaded > apps. For what it's worth, all reasonable STL implementations will use a > lightweight mutex implementation, which generally require less than 100 > nanoseconds to acquire uncontended, so I think we don't need to care, but > again some feedback would be nice. >I would really like to move LLVM to a point where it never had a mutex lock acquisition in such a critical path that removing it for single threaded users was important. Any such use of a mutex is, IMO, a performance bug for *both* the single threaded users and the multithreaded users! We shouldn't hack around this by making a synchronization construct that isn't really a synchronization construct. That makes LLVM increasingly hard to develop for very marginal gains. Note that I think your 100 nanosecond estimate is really conservative. There are many mutex implementations even faster. I use the numbers also cited by Jeff Dean and Peter Norvig which estimate it at 25 nanoseconds: http://norvig.com/21-days.html#answers> 3) We have some debug code in our mutex implementation that is intended to > try to help catch deadlocks and/or race conditions. Do we need this code? > > I think we can get by without it, but again some feedback about how, if at > all, people are using this would be nice. For example, if you want to > detect deadlocks and race conditions you can use TSan. >I think that we should add such debug code to the std::mutex implementation in libc++, and then most developers should have easy access to it. I'll talk to Kostya and the folks that work on TSan to see if they would be up for contributing it. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140606/15b2026f/attachment.html>
On Sat, Jun 7, 2014 at 5:47 AM, Zachary Turner <zturner at google.com> wrote:> +chandlerc, aaronballman, in case there are additional carryovers and/or > issues from the review thread which I've left out. > > > I have a patch up for review[1, 2] that attempts to replace LLVM's mutex > implementation with std::mutex and std::recursive_mutex. While the patch > seems to work, there are questions surrounding whether or not the approach > used is correct. > > I'll try to summarize the issues as best I can, in hopes of getting some > feedback from a broader audience, to make sure this is done correctly: > > 1) Should support multi-threading be a compile-time or runtime parameter > in LLVM? > > Currently it is both. It is compile-time through the use of the > define LLVM_ENABLE_THREADS, and it is runtime through the use of functions > llvm_start_multithreaded, llvm_is_multithreaded, etc. I and some others > feel like runtime support for multi-threading could be removed, and it > should be compile-time only. However, I am not aware of all the ways in > which this is being used, so this is where I would like some feedback. The > issues I have with runtime multithreading support are the following: > > * It leads to confusing code. At any given point, is multi-threading > enabled or disabled? You never know without calling llvm_is_multithreaded, > but even calling that is inherently racy, because someone else could > disable it after it returns. > > * It leads to subtle bugs. clang_createIndex, the first time it's called, > enables multi-threading. What happens if someone else disables it later? > Things like this shouldn't even be possible. > > * Not all platforms even support threading to begin with. This works now > because llvm_start_multithreaded(), if the compile time flag is set to > disable threads, simply does nothing. But this decision should be made by > someone else. Nobody really checks the return value from > llvm_start_multithreaded anyway, so there's already probably bugs where > someone tries to start multi-threading, and it fails. > > * What does it actually mean to turn multi-threading support on and off? > > Anybody that tries to do this is almost certainly broken due to some edge > cases about when it's on and when it's off. So this goes back to the first > two points about confusing code and subtle bugs. >+1 I am in favor of removing the run-time check. We've seen lots of code like this "if (have_threads) mu->lock();", and it was very often broken.> > 2) What should happen when you try to acquire a mutex in an app with > threading disabled? > > If this is a compile-time parameter, the solution is simple: make an empty > mutex class that satisfies the Lockable concept, and have its methods do > nothing. Then typedef something like llvm::mutex to be either std::mutex > or llvm::null_mutex accordingly. If this is a runtime parameter, it's more > complicated, and we should ask ourselves whether or not it's necessary to > avoid the overhead of acquiring an uncontended mutex in single threaded > apps. For what it's worth, all reasonable STL implementations will use a > lightweight mutex implementation, which generally require less than 100 > nanoseconds to acquire uncontended, so I think we don't need to care, but > again some feedback would be nice. > > 3) We have some debug code in our mutex implementation that is intended to > try to help catch deadlocks and/or race conditions. Do we need this code? > > I think we can get by without it, but again some feedback about how, if at > all, people are using this would be nice. For example, if you want to > detect deadlocks and race conditions you can use TSan. >The problem with race detection is that it slows down the process, but I doubt that a debug code in mutex can catch (many) races. As for the deadlocks, indeed it is possible to add deadlock detection directly to std::mutex and std::spinlock code. It may even end up being more efficient than a standalone deadlock detector -- but only if we can add an extra word to the mutex/spinlock object. The deadlock detector's overhead comes primarily from two tasks: 1. get the metadata for the lock in question. 2. query the lock-acquisition-order graph to see if there is a loop. If the lock-acquisition-order graph is sparse (99% of cases we've seen), then the task #1 may constitute up to 50% of the overhead. If we can add a word to std::mutex/std::spinlock data structures then the task #1 becomes trivial. If we can't add a word for some reason, then I see no benefit from adding debug mode directly to STL compared to a standalone deadlock detector. (FTR: our deadlock detector in tsan: https://code.google.com/p/thread-sanitizer/wiki/DeadlockDetector) --kcc> > Thanks! > Zach > > > [1] - > http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140602/220051.html > [2] - http://reviews.llvm.org/D4033 > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140607/02bf703f/attachment.html>
On Fri, Jun 6, 2014 at 10:57 PM, Kostya Serebryany <kcc at google.com> wrote:> As for the deadlocks, indeed it is possible to add deadlock detection > directly to std::mutex and std::spinlock code. > It may even end up being more efficient than a standalone deadlock > detector -- > but only if we can add an extra word to the mutex/spinlock object. > The deadlock detector's overhead comes primarily from two tasks: > 1. get the metadata for the lock in question. > 2. query the lock-acquisition-order graph to see if there is a loop. > > If the lock-acquisition-order graph is sparse (99% of cases we've seen), > then the task #1 may constitute up to 50% of the overhead. > If we can add a word to std::mutex/std::spinlock data structures then the > task #1 becomes trivial. >I don't see any reason not to reserve a word in the mutex so that in (an ABI-compatible) debug build the mutex can support deadlock detection. Some people are super concerned about having an extra word in a mutex, but I'm not at all. For libc++, it would probably need to be behind an ABI-breaking macro on Mac and FreeBSD, but we haven't committed to any ABI stability on Linux, so we could probably enable it by default there, and get into build bots. Maybe bring this up on the cfe-dev list to discuss with Marshall and other folks interested in libc++? -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140606/fa94e597/attachment.html>
On Fri, Jun 6, 2014 at 9:47 PM, Zachary Turner <zturner at google.com> wrote:> +chandlerc, aaronballman, in case there are additional carryovers and/or > issues from the review thread which I've left out. > > > I have a patch up for review[1, 2] that attempts to replace LLVM's mutex > implementation with std::mutex and std::recursive_mutex. While the patch > seems to work, there are questions surrounding whether or not the approach > used is correct. > > I'll try to summarize the issues as best I can, in hopes of getting some > feedback from a broader audience, to make sure this is done correctly: > > 1) Should support multi-threading be a compile-time or runtime parameter in > LLVM? > > Currently it is both. It is compile-time through the use of the define > LLVM_ENABLE_THREADS, and it is runtime through the use of functions > llvm_start_multithreaded, llvm_is_multithreaded, etc. I and some others > feel like runtime support for multi-threading could be removed, and it > should be compile-time only. However, I am not aware of all the ways in > which this is being used, so this is where I would like some feedback. The > issues I have with runtime multithreading support are the following: > > * It leads to confusing code. At any given point, is multi-threading > enabled or disabled? You never know without calling llvm_is_multithreaded, > but even calling that is inherently racy, because someone else could disable > it after it returns. > > * It leads to subtle bugs. clang_createIndex, the first time it's called, > enables multi-threading. What happens if someone else disables it later? > Things like this shouldn't even be possible. > > * Not all platforms even support threading to begin with. This works now > because llvm_start_multithreaded(), if the compile time flag is set to > disable threads, simply does nothing. But this decision should be made by > someone else. Nobody really checks the return value from > llvm_start_multithreaded anyway, so there's already probably bugs where > someone tries to start multi-threading, and it fails. > > * What does it actually mean to turn multi-threading support on and off? > > Anybody that tries to do this is almost certainly broken due to some edge > cases about when it's on and when it's off. So this goes back to the first > two points about confusing code and subtle bugs.I agree with your assessment, and my personal feeling is that this should be a compile-time switch. Code which attempts to do this at runtime often has subtle bugs.> > 2) What should happen when you try to acquire a mutex in an app with > threading disabled? > > If this is a compile-time parameter, the solution is simple: make an empty > mutex class that satisfies the Lockable concept, and have its methods do > nothing. Then typedef something like llvm::mutex to be either std::mutex or > llvm::null_mutex accordingly. If this is a runtime parameter, it's more > complicated, and we should ask ourselves whether or not it's necessary to > avoid the overhead of acquiring an uncontended mutex in single threaded > apps. For what it's worth, all reasonable STL implementations will use a > lightweight mutex implementation, which generally require less than 100 > nanoseconds to acquire uncontended, so I think we don't need to care, but > again some feedback would be nice.I agree with the compile time approach.> > 3) We have some debug code in our mutex implementation that is intended to > try to help catch deadlocks and/or race conditions. Do we need this code? > > I think we can get by without it, but again some feedback about how, if at > all, people are using this would be nice. For example, if you want to > detect deadlocks and race conditions you can use TSan.I don't think the code is needed (I believe TSan suffices), but is it possible that there are platforms for which TSan is unsupported and this code may prove useful? I'm not certain. As Chandler points out, I think supporting this at the libc++ level may actually be more profitable. ~Aaron
Hi, On Saturday, June 07, 2014 10:26:29 Aaron Ballman wrote:> > 1) Should support multi-threading be a compile-time or runtime parameter in > > LLVM? > > > > Currently it is both. It is compile-time through the use of the define > > LLVM_ENABLE_THREADS, and it is runtime through the use of functions > > llvm_start_multithreaded, llvm_is_multithreaded, etc. I and some others > > feel like runtime support for multi-threading could be removed, and it > > should be compile-time only. However, I am not aware of all the ways in > > which this is being used, so this is where I would like some feedback. The > > issues I have with runtime multithreading support are the following: > > > > * It leads to confusing code. At any given point, is multi-threading > > enabled or disabled? You never know without calling llvm_is_multithreaded, > > but even calling that is inherently racy, because someone else could disable > > it after it returns. > > > > * It leads to subtle bugs. clang_createIndex, the first time it's called, > > enables multi-threading. What happens if someone else disables it later? > > Things like this shouldn't even be possible. > > > > * Not all platforms even support threading to begin with. This works now > > because llvm_start_multithreaded(), if the compile time flag is set to > > disable threads, simply does nothing. But this decision should be made by > > someone else. Nobody really checks the return value from > > llvm_start_multithreaded anyway, so there's already probably bugs where > > someone tries to start multi-threading, and it fails. > > > > * What does it actually mean to turn multi-threading support on and off? > > > > Anybody that tries to do this is almost certainly broken due to some edge > > cases about when it's on and when it's off. So this goes back to the first > > two points about confusing code and subtle bugs. > > I agree with your assessment, and my personal feeling is that this > should be a compile-time switch. Code which attempts to do this at > runtime often has subtle bugs.Just to make you aware of a use case scenario that strikes me since some time together with the open source OpenGL driver stack that makes use of llvm. There you see driver modules which potentially get loaded anywhere at application run time. Potentially this can happen from any thread (even if in reality probably serialized by the dynamic loader) and thus can also race against the main application itself calling llvm_start_multithreaded in an other thread. Or if I remember correctly as documented, llvm_start_multithreaded must not be called concurrently to any other llvm api calls possibly accessing any of the llvm singeltons. Which in turn is hard to guarantee if you get dynamically loaded at run time and you have to expect the main application to use llvm already. Given that I would wish that you can just make use of llvm concurrently and having that no option at all, but I can easily anticipate the usual resistance against that. Nevertheless if multithreading is a compile option, it would be great if a user of llvm can see if the llvm libraries have been compiled with multithreading enabled or not. Probably best queriable at runtime so that a potential user can bail out in a nice way instead of just crashing later ... So it would be good if you keep this use case in mind when doing changes in this corner! Thanks Mathias