Messages in this thread Patch in this message | | | Date | Wed, 14 Apr 2010 21:11:44 +0200 | From | Manfred Spraul <> | Subject | Re: [PATCH 1/2] ipc semaphores: reduce ipc_lock contention in semtimedop |
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On 04/14/2010 07:33 PM, Chris Mason wrote: > On Wed, Apr 14, 2010 at 06:16:53PM +0200, Manfred Spraul wrote: > >> On 04/13/2010 08:19 PM, Chris Mason wrote: >> >>> On Wed, Apr 14, 2010 at 04:09:45AM +1000, Nick Piggin wrote: >>> >>>> On Tue, Apr 13, 2010 at 01:39:41PM -0400, Chris Mason wrote: >>>> The other thing I don't know if your patch gets right is requeueing on >>>> of the operations. When you requeue from one list to another, then you >>>> seem to lose ordering with other pending operations, so that would >>>> seem to break the API as well (can't remember if the API strictly >>>> mandates FIFO, but anyway it can open up starvation cases). >>>> >>> I don't see anything in the docs about the FIFO order. I could add an >>> extra sort on sequence number pretty easily, but is the starvation case >>> really that bad? >>> >>> >> How do you want to determine the sequence number? >> Is atomic_inc_return() on a per-semaphore array counter sufficiently fast? >> > I haven't tried yet, but hopefully it won't be a problem. A later patch > does atomics on the reference count and it doesn't show up in the > profiles. > > >> >>>> I was looking at doing a sequence number to be able to sort these, but >>>> it ended up getting over complex (and SAP was only using simple ops so >>>> it didn't seem to need much better). >>>> >>>> We want to be careful not to change semantics at all. And it gets >>>> tricky quickly :( What about Zach's simpler wakeup API? >>>> >>> Yeah, that's why my patches include code to handle userland sending >>> duplicate semids. Zach's simpler API is cooking too, but if I can get >>> this done without insane complexity it helps with more than just the >>> post/wait oracle workload. >>> >>> >> What is the oracle workload, which multi-sembuf operations does it use? >> How many semaphores are in one array? >> >> When the last optimizations were written, I've searched a bit: >> - postgres uses per-process semaphores, with small semaphore arrays. >> [process sleeps on it's own semaphore and is woken up by someone >> else when it can make progress] >> > This is similar to Oracle (and the sembench program). Each process has > a semaphore and when it is waiting for a commit it goes to sleep on it. > They are woken up in bulk with semtimedop calls from a single process. > > Hmm. Thus you have: - single sembuf decrease operations that are waiting frequently. - multi-sembuf increase operations.
What about optimizing for that case? Increase operations succeed immediately. Thus complex_count is 0.
If we have performed an update operation, then we can scan all simple_lists that have seen an increase instead of checking the global list - as long as there are no complex operations waiting. Right now, we give up if the update operation was a complex operation - but that does not matter. All that matters are the sleeping operations, not the operation that did the wakeup. I've attached an untested idea.
> But oracle also uses semaphores for locking in a traditional sense. > > Putting the waiters into a per-semaphore list is really only part of the > speedup. The real boost comes from the patch to break up the locks into > a per semaphore lock. > > Ok. Then simple tricks won't help. How many semaphores are in one array?
-- Manfred diff --git a/ipc/sem.c b/ipc/sem.c index dbef95b..8986239 100644 --- a/ipc/sem.c +++ b/ipc/sem.c @@ -1224,8 +1224,18 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops, error = try_atomic_semop (sma, sops, nsops, un, task_tgid_vnr(current)); if (error <= 0) { - if (alter && error == 0) - update_queue(sma, (nsops == 1) ? sops[0].sem_num : -1); + if (alter && error == 0) { + if (sma->complex_count) { + update_queue(sma, -1); + } else { + int i; + + for (i=0;i<nsops;i++) { + if (sops[i].sem_op > 0) + update_queue(sma, sops[i].sem_num); + } + } + } goto out_unlock_free; } | |