Re: [rfc] direct IO submission and completion scalability issues

[Jens Axboe]

On Sun, Feb 03 2008, Nick Piggin wrote:
> On Fri, Jul 27, 2007 at 06:21:28PM -0700, Suresh B wrote:
> > 
> > Second experiment which we did was migrating the IO submission to the
> > IO completion cpu. Instead of submitting the IO on the same cpu where the
> > request arrived, in this experiment  the IO submission gets migrated to the
> > cpu that is processing IO completions(interrupt). This will minimize the
> > access to remote cachelines (that happens in timers, slab, scsi layers). The
> > IO submission request is forwarded to the kblockd thread on the cpu receiving
> > the interrupts. As part of this, we also made kblockd thread on each cpu as the
> > highest priority thread, so that IO gets submitted as soon as possible on the
> > interrupt cpu with out any delay. On x86_64 SMP platform with 16 cores, this
> > resulted in 2% performance improvement and 3.3% improvement on two node ia64
> > platform.
> > 
> > Quick and dirty prototype patch(not meant for inclusion) for this io migration
> > experiment is appended to this e-mail.
> > 
> > Observation #1 mentioned above is also applicable to this experiment. CPU's
> > processing interrupts will now have to cater IO submission/processing
> > load aswell.
> > 
> > Observation #2: This introduces some migration overhead during IO submission.
> > With the current prototype, every incoming IO request results in an IPI and
> > context switch(to kblockd thread) on the interrupt processing cpu.
> > This issue needs to be addressed and main challenge to address is
> > the efficient mechanism of doing this IO migration(how much batching to do and
> > when to send the migrate request?), so that we don't delay the IO much and at
> > the same point, don't cause much overhead during migration.
> 
> Hi guys,
> 
> Just had another way we might do this. Migrate the completions out to
> the submitting CPUs rather than migrate submission into the completing
> CPU.
> 
> I've got a basic patch that passes some stress testing. It seems fairly
> simple to do at the block layer, and the bulk of the patch involves
> introducing a scalable smp_call_function for it.
> 
> Now it could be optimised more by looking at batching up IPIs or
> optimising the call function path or even mirating the completion event
> at a different level...
> 
> However, this is a first cut. It actually seems like it might be taking
> slightly more CPU to process block IO (~0.2%)... however, this is on my
> dual core system that shares an llc, which means that there are very few
> cache benefits to the migration, but non-zero overhead. So on multisocket
> systems hopefully it might get to positive territory.

That's pretty funny, I did pretty much the exact same thing last week!
The primary difference between yours and mine is that I used a more
private interface to signal a softirq raise on another CPU, instead of
allocating call data and exposing a generic interface. That put the
locking in blk-core instead, turning blk_cpu_done into a structure with
a lock and list_head instead of just being a list head, and intercepted
at blk_complete_request() time instead of waiting for an already raised
softirq on that CPU.

Didn't get around to any performance testing yet, though. Will try and
clean it up a bit and do that.

-- 
Jens Axboe

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