Messages in this thread | | | Date | Wed, 14 Apr 2010 13:33:19 -0400 | From | Chris Mason <> | Subject | Re: [PATCH 1/2] ipc semaphores: reduce ipc_lock contention in semtimedop |
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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.
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.
We gain another 10-15% from a later patch that gets uses atomics on the refcount, which lets us do sem_putref without a lock (meaning we're lockless once we get woken up).
I'm cleaning up fixes based on suggestions here and will repost.
> - with google, I couldn't find anything relevant that uses > multi-sembuf semop() calls. >
I think this should help any workload that has more than one semaphore per array, even if they only do one sem per call.
> And I agree with Nick: We should be careful about changing the API.
Definitely, thanks for reading through it.
-chris
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