linux-kernel - Re: [PATCH 1/2] ipc semaphores: reduce ipc

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Date:	Wed, 14 Apr 2010 18:16:53 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>, zach.brown@...cle.com,
	jens.axboe@...cle.com, linux-kernel@...r.kernel.org
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
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