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

Nick Piggin
2010-05-18T06:23:49Z

On Sun, May 16, 2010 at 06:57:38PM +0200, Manfred Spraul wrote:
> On 04/13/2010 08:57 PM, Nick Piggin wrote:
> >On Tue, Apr 13, 2010 at 02:19:37PM -0400, Chris Mason wrote:
> >>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.
> >
> The management process won't get the sem on Linux either:
> Linux implements FIFO, but there is no protection at all against starvation.

Yeah I did realise this after I posted. But anyway I think FIFO
is reasonable to have, although you *may* be able to justify
removing it after your research of other UNIXes, if there are
sufficient gains.

>
> If I understand the benchmark numbers correctly, a 4-core, 2 GHz
> Phenom is able to do ~ 2 million semaphore operations per second in
> one semaphore array.
> That's the limit - cache line trashing on the sma structure prevent
> higher numbers.
>
> For a NUMA system, the limit is probably lower.
>
> Chris:
> Do you have an estimate how many semop() your app will perform in one array?
>
> Perhaps we should really remove the per-array list,
> sma->sem_perm.lock and sma->sem_otime.
>
> --
> Manfred