Re: [PATCH v2 0/4] ipc: reduce ipc lock contention

[Dave Kleikamp <dave.kleikamp@oracle.com>]

On 03/07/2013 06:55 AM, Chris Mason wrote:
> On Thu, Mar 07, 2013 at 01:45:33AM -0700, Peter Zijlstra wrote:
>> On Tue, 2013-03-05 at 15:53 -0500, Rik van Riel wrote:
>>
>>> Indeed.  Though how well my patches will work with Oracle will
>>> depend a lot on what kind of semctl syscalls they are doing.
>>>
>>> Does Oracle typically do one semop per semctl syscall, or does
>>> it pass in a whole bunch at once?
>>
>> https://oss.oracle.com/~mason/sembench.c
>>
>> I think Chris wrote that to match a particular pattern of semaphore
>> operations the database engine in question does. I haven't checked to
>> see if it triggers the case in point though.
>>
>> Also, Chris since left Oracle but maybe he knows who to poke.
>>
> 
> Dave Kleikamp (cc'd) took over my patches and did the most recent
> benchmarking.  Ported against 3.0:
> 
> https://oss.oracle.com/git/?p=linux-uek-2.6.39.git;a=commit;h=c7fa322dd72b08450a440ef800124705a1fa148c
> 
> The current versions are still in the 2.6.32 oracle kernel, but it looks
> like they reverted this 3.0 commit.  I think with Manfred's upstream
> work my more complex approach wasn't required anymore, but hopefully
> Dave can fill in details.

>From what I recall, I could never get better performance from your
patches that we saw with Manfred's work alone. I can't remember the
reasons for including and then reverting the patches from the 3.0
(2.6.39) Oracle kernel, but in the end we weren't able to justify their
inclusion.

> Here is some of the original discussion around the patch:
> 
> https://lkml.org/lkml/2010/4/12/257
> 
> In terms of how oracle uses IPC, the part that shows up in profiles is
> using semtimedop for bulk wakeups.  They can configure things to use
> either a bunch of small arrays or a huge single array (and anything in
> between). 
> 
> There is one IPC semaphore per process and they use this to wait for
> some event (like a log commit).  When the event comes in, everyone
> waiting is woken in bulk via a semtimedop call.
> 
> So, single proc waking many waiters at once.
> 
> -chris
>