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

[Chris Mason]

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.

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

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