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

Manfred Spraul
2010-04-14T16:16:51Z

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