Messages in this thread | | | Date | Thu, 4 Mar 2010 11:03:09 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH -tip] introduce sys_membarrier(): process-wide memory barrier (v9) |
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* Mathieu Desnoyers (mathieu.desnoyers@efficios.com) wrote: > * Ingo Molnar (mingo@elte.hu) wrote: > > > > * Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote: > > > > > I am proposing this patch for the 2.6.34 merge window, as I think it is > > > ready for inclusion. > > > > It's a bit late for this merge window i think. > > OK, no problem. Thanks for taking time to review the patch. See below for > response to your comments. > > > > > > Here is an implementation of a new system call, sys_membarrier(), which > > > executes a memory barrier on all threads of the current process. It can be > > > used to distribute the cost of user-space memory barriers asymmetrically by > > > transforming pairs of memory barriers into pairs consisting of > > > sys_membarrier() and a compiler barrier. For synchronization primitives that > > > distinguish between read-side and write-side (e.g. userspace RCU, rwlocks), > > > the read-side can be accelerated significantly by moving the bulk of the > > > memory barrier overhead to the write-side. > > > > Why is this such a low level and still special-purpose facility? > > > > Synchronization facilities for high-performance threading may want to do a bit > > more than just execute a barrier instruction on another CPU that has a > > relevant thread running. > > Yep, I'm aware of that. > > > > > You cited signal based numbers: > > > > > (what we have now, with dynamic sys_membarrier check, expedited scheme) > > > memory barriers in reader: 907693804 reads, 817793 writes > > > sys_membarrier scheme: 4316818891 reads, 503790 writes > > > > > > (dynamic sys_membarrier check, non-expedited scheme) > > > memory barriers in reader: 907693804 reads, 817793 writes > > > sys_membarrier scheme: 8698725501 reads, 313 writes > > > > Much of that signal handler overhead is i think due to: > > > > - FPU/SSE context save/restore > > - the need to wake up, run and deschedule all threads > > This second point hurts, especially if we have more threads than processors. > > > > > Instead i'd suggest for you to try to implement user-space RCU speedups not > > via the new sys_membarrier() syscall, but via two new signal extensions: > > > > - SA_NOFPU: on x86 to skip the FPU/SSE save/restore, for such fast in/out special > > purpose signal handlers? (can whip up a quick patch for you if you want) > > This could help. > > > > > - SA_RUNNING: a way to signal only running threads - as a way for user-space > > based concurrency control mechanisms to deschedule running threads (or, like > > in your case, to implement barrier / garbage collection schemes). > > > > ( Note: to properly sync back you'll also need an sa_info field to tell > > target tasks how many tasks were woken up. That way a futex can be used > > as a semaphore to signal back to the issuing thread, and make it all > > properly event triggered and nicely scalable. Also, queued signals are a > > must for such a scheme. ) > > Ah, nice! I wondered how you'd propose to deal with that one. It was actually my > main problem: how to wait for all running threads to complete their execution. > This added sa_info count and futex usage will indeed deal with the problem. And > rt_sigqueueinfo() will ensure that we don't collapse multiple concurrent > requests for execution of the same signal. For syncing back, I think we can do > this without modifying sa_info. Simply passing a pointer to the counter to > increment in the sigval value to rt_sigqueueinfo() should do the trick.
Hrm, I overlooked the fact that this counter must be written by the signal sender. So we probably need to add a field to sa_info as you proposed.
Thanks,
Mathieu
> > > > > My estimation is that it will be _much_ faster than the naive signal based > > approach - maybe even quite comparable to an open-coded sys_membarrier(): > > Yes, especially given that your proposal permits to send all signals in in > "broadcast to all running threads" mode, in a single system call. > > > > > - as most of the overhead in a real scenario ought to be the IPI sending and > > latency - not the syscall entry/exit. (with a signal approach we'd still go > > into target thread user-mode, so one more syscall exit+re-entry) > > > > - or for the common case where there are no other threads running, we are > > just in/out of SA_RUNNING without having to do any synchronization. In that > > case it should be quite close to sys_membarrier() - modulo some minimal > > signal API overhead. [which we could optimize some more, if it's visible in > > your benchmarks.] > > > > Signals per se are pretty scalable these days - now that most of the fastpaths > > are decoupled from tasklist_lock and everything is RCU-ized. > > > > Further benefits are: > > > > - both SA_NOFPU and SA_RUNNING could be used by a _lot_ more user-space > > facilities than just user-space RCU. > > > > - synergetic effects: growing some real high-performance facility based on > > signals would ensure further signal speedups in the future as well. > > Currently any server app that runs into signal limitations tends to shy > > away from them and use some different (and often inferior) signalling > > scheme. It would be better extend signals with 'lightweight' capabilities > > as well. > > > > All in one, signals are used by like 99.9% of Linux apps, while > > sys_membarrier() would be used only by [WAG] 0.00001% of them. > > > > So before we can merge this (at least via the RCU tree, which you have sent it > > to), i'd like to see you try _much_, _MUCH_ harder to fix the very obvious > > signal overhead performance problems you have demoed via the numbers above so > > nicely. > > I think we can start with the SA_RUNNING+modified sa_info approach to signal > only running threads. I expect that much of the benefit will come from there. > Then, from that point, we can see if SA_NOFPU provides a significant performance > improvement. > > Now, a very basic questions: in the signal-based approach I currently use, I > reserve SIGUSR1 _from my liburcu library_ (yeah, that's pretty ugly). The > problem is: how can I reserve new signal numbers from a library point of view > without having the applications using it too ? We have room left in the rt > signals numbers, so maybe this is a lesser problem than with standard signals, > which are quite full, but the problem of making sure the application does not > conflict stays. > > > > > If _that_ fails, and if we get all the fruits of that, _then_ we might > > perhaps, with a lot of hesitation, concede defeat and think about adding yet > > another syscall. > > > > I know it's cool to add a brand new syscall - but, unfortunately, in practice > > it doesnt help Linux apps all that much. (at least until we have tools/klibc/ > > or so.) > > > > [ There's also a few small cleanliness details i noticed in your patch: enums > > are a tiny bit nicer for ABIs than #define's, the #ifdef SMP is ugly, etc. - > > but it doesnt really matter much as i think we should concentrate on the > > scalability problems of signals first. ] > > OK, let's do that. > > Thanks, > > Mathieu > > > > > Thanks, > > > > Ingo > > -- > Mathieu Desnoyers > Operating System Efficiency Consultant > EfficiOS Inc. > http://www.efficios.com
-- Mathieu Desnoyers Operating System Efficiency Consultant EfficiOS Inc. http://www.efficios.com
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