Messages in this thread | | | Date | Wed, 14 Apr 2010 18:16:53 +0200 | From | Manfred Spraul <> | Subject | Re: [PATCH 1/2] ipc semaphores: reduce ipc_lock contention in semtimedop |
| |
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 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] - with google, I couldn't find anything relevant that uses multi-sembuf semop() calls.
And I agree with Nick: We should be careful about changing the API.
-- Manfred
| |