Re: [PATCH 1/2] ipc semaphores: reduce ipc_lock contention in semtimedop

Chris Mason
2010-04-13T19:02:12Z

On Wed, Apr 14, 2010 at 04:57:56AM +1000, Nick Piggin wrote:
> On Tue, Apr 13, 2010 at 02:19:37PM -0400, 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:
> > > > On Tue, Apr 13, 2010 at 07:15:30PM +0200, Manfred Spraul wrote:
> > > > > Hi Chris,
> > > > >
> > > > >
> > > > > On 04/12/2010 08:49 PM, Chris Mason wrote:
> > > > > > /*
> > > > > >+ * when a semaphore is modified, we want to retry the series of operations
> > > > > >+ * for anyone that was blocking on that semaphore. This breaks down into
> > > > > >+ * a few different common operations:
> > > > > >+ *
> > > > > >+ * 1) One modification releases one or more waiters for zero.
> > > > > >+ * 2) Many waiters are trying to get a single lock, only one will get it.
> > > > > >+ * 3) Many modifications to the count will succeed.
> > > > > >+ *
> > > > > Have you thought about odd corner cases:
> > > > > Nick noticed the last time that it is possible to wait for arbitrary values:
> > > > > in one semop:
> > > > > - decrease semaphore 5 by 10
> > > > > - wait until semaphore 5 is 0
> > > > > - increase semaphore 5 by 10.
> > > >
> > > > Do you mean within a single sop array doing all three of these? I don't
> > > > know if the sort is going to leave the three operations on semaphore 5
> > > > in the same order (it probably won't).
> > > >
> > > > But I could change that by having it include the slot in the original
> > > > sop array in the sorting. That way if we have duplicate semnums in the
> > > > array, they will end up in the same position relative to each other in
> > > > the sorted result.
> > > >
> > > > (ewwww ;)
> > >
> > > I had a bit of a hack at doing per-semaphore stuff when I was looking
> > > at the first optimization, but it was tricky to make it work.
> > >
> > > 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?
>
> Yes, because it's not just a theoretical livelock, it can be basically
> a certainty, given the right pattern of semops.
>
> You could have two mostly-independent groups of processes, each taking
> and releasing a different sem, which are always contended (eg. if it is
> being used for a producer-consumer type situation, or even just mutual
> exclusion with high contention).
>
> Then you could have some overall management process for example which
> tries to take both sems. It will never get it.

Ok, fair enough, I'll add the sequence number.

>
>
> > > 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.
>
> Duplicate semids? What do you mean?

Sorry, semnums...index into the array of semaphores.

>
>
> > 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.
>
> Iam worried about complexity and slowing other cases, given that Oracle
> DB seems willing to adapt to the (better suited) new API. So I'd be
> interested to know what it helps outside Oracle.
>

Sure, I'd hope that your benchmark from last time around is faster now.

-chris