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SubjectRe: io_request_lock patch?
Hi Jens,

On Wed, Jul 11, 2001 at 09:19:00AM +0200, Jens Axboe wrote:
> On Tue, Jul 10 2001, Dipankar Sarma wrote:
> > In article <20010709214453.U16505@suse.de> you wrote:
> > > On Mon, Jul 09 2001, Jonathan Lahr wrote:
> >
> > SPINLOCKS HOLD WAIT
> > UTIL CON MEAN( MAX ) MEAN( MAX )(% CPU) TOTAL NOWAIT SPIN RJECT NAME
> >
> > 0.07% 60.2% 0.5us( 4.0us) 72us( 920us)( 1.7%) 610820 39.8% 60.2% 0% blk_get_queue+0x10
> >
> > 1557496*26.7%*72us makes it about 30 seconds of time waiting for
> > io_request_lock. That is nearly one-third of the total system time
> > (about 98 seconds). As number of CPUs increase, this will likely
> > worsen.
>
> Auch! That's pretty bad.
>
> > It also seems that __make_request() holds the lock for the largest
> > amount of time. This hold time isn't likely to change significantly
>
> __make_request -> elevator merge/insertion scan. This is what is taking
> all the time, not __make_request itself. With the bio-XX patches I have
> completely eliminated merge scans, so that can be done in O(1) time. For
> now insertion is still a O(N) scan, maybe that will change too [1].

I haven't got as far down as the elevator algorithm yet, but I would
like to, at some point in time. In any case, my point was that because
of disk block sorting done during initial queueing, there is likely
to be a slightly longer lock (per-queue or otherwise) hold time there compared
to, say, dequeueing for dispatch to lowlevel drivers.

Where can I get the bio patches from ?

>
> > for a per-queue lock, but atleast it will not affect queueing i/o
> > requests to other devices. Besides, I am not sure if blk_get_queue()
> > really needs to grab the io_request_lock. blk_dev[] entries aren't
>
> Funny, this is one thing I've been looking at too. blk_get_queue _will_
> die, don't worry. And yes, ie at open we can assign the queue. Or simply
> map it and protect it otherwise. It all ties in with being able to up or
> down a queue too, currently grabbing io_request_lock from blk_get_queue
> accomplishes exactly nothing and may as well be removed. If you do that,
> does it change the contention numbers significantly?

I haven't yet experimented with this yet, but theoritically speaking
yes, it should make a big difference. blk_get_queue() grabs the lock
very often and holds it for a very short period of time on average,
so it is the one that is affected most. Out of the 30 seconds of
spin-wait for io_request_lock, blk_get_queue() seems to take up
610820*60.2%*72us = 26.5 seconds. I will get to this soon though.

BTW, where can I get some of these lock-splitting patches from ? I
can do one myself for scsi+aic7xxx, but if there already exist some
work, I would like to start off with them.

Thanks
Dipankar
--
Dipankar Sarma <dipankar@sequent.com> Project: http://lse.sourceforge.net
Linux Technology Center, IBM Software Lab, Bangalore, India.
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