Messages in this thread | | | Date | Mon, 17 May 2010 17:20:13 +1000 | From | Nick Piggin <> | Subject | Re: [PATCH 1/2] ipc semaphores: reduce ipc_lock contention in semtimedop |
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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
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