Message-ID: <20100112223638.GH3065@redhat.com>
Date:	Tue, 12 Jan 2010 17:36:38 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Jeff Moyer <jmoyer@...hat.com>,
	Shaohua Li <shaohua.li@...el.com>,
	Gui Jianfeng <guijianfeng@...fujitsu.com>,
	Yanmin Zhang <yanmin_zhang@...ux.intel.com>
Subject: Re: [PATCH] cfq-iosched: rework seeky detection

On Tue, Jan 12, 2010 at 09:05:29PM +0100, Corrado Zoccolo wrote:
> Hi Vivek,
> On Tue, Jan 12, 2010 at 8:12 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
> > On Sat, Jan 09, 2010 at 04:59:17PM +0100, Corrado Zoccolo wrote:
> >> Current seeky detection is based on average seek lenght.
> >> This is suboptimal, since the average will not distinguish between:
> >> * a process doing medium sized seeks
> >> * a process doing some sequential requests interleaved with larger seeks
> >> and even a medium seek can take lot of time, if the requested sector
> >> happens to be behind the disk head in the rotation (50% probability).
> >>
> >> Therefore, we change the seeky queue detection to work as follows:
> >> * each request can be classified as sequential if it is very close to
> >>   the current head position, i.e. it is likely in the disk cache (disks
> >>   usually read more data than requested, and put it in cache for
> >>   subsequent reads). Otherwise, the request is classified as seeky.
> >> * an history window of the last 32 requests is kept, storing the
> >>   classification result.
> >> * A queue is marked as seeky if more than 1/8 of the last 32 requests
> >>   were seeky.
> >>
> >> This patch fixes a regression reported by Yanmin, on mmap 64k random
> >> reads.
> >>
> >
> > Ok, I did basic testing of this patch on my hardware. I got a RAID-0
> > configuration and there are 12 disks behind it. I ran 8 fio mmap random
> > read processes with block size 64K and following are the results.
> >
> > Vanilla (3 runs)
> > ===============
> > aggrb=3,564KB/s (cfq)
> > aggrb=3,600KB/s (cfq)
> > aggrb=3,607KB/s (cfq)
> >
> > aggrb=3,992KB/s,(deadline)
> > aggrb=3,953KB/s (deadline)
> > aggrb=3,991KB/s (deadline)
> >
> > Patched kernel (3 runs)
> > =======================
> > aggrb=2,080KB/s (cfq)
> > aggrb=2,100KB/s (cfq)
> > aggrb=2,124KB/s (cfq)
> >
> > My fio script
> > =============
> > [global]
> > directory=/mnt/sda/fio/
> > size=8G
> > direct=0
> > runtime=30
> > ioscheduler=cfq
> > exec_prerun="echo 3 > /proc/sys/vm/drop_caches"
> > group_reporting=1
> > ioengine=mmap
> > rw=randread
> > bs=64K
> >
> > [randread]
> > numjobs=8
> > =================================
> >
> > There seems to be around more than 45% regression in this case.
> >
> > I have not run the blktrace, but I suspect it must be coming from the fact
> > that we are now treating a random queue as sync-idle hence driving queue
> > depth as 1. But the fact is that read ahead much not be kicking in, so
> > we are not using the power of parallel processing this striped set of
> > disks can do for us.
> >
> Yes. Those results are expected, and are the other side of the medal.
> If we handle those queues as sync-idle, we get better performance
> on single disk (and regression on RAIDs), and viceversa if we handle
> them as sync-noidle.
> 
> Note that this is limited to mmap with large block size. Normal read/pread
> is not affected.
> 
> > So treating this kind of cfqq as sync-idle seems to be a bad idea atleast
> > on configurations where multiple disks are in raid configuration.
> 
> The fact is, can we reliably determine which of those two setups we
> have from cfq?

I have no idea at this point of time but it looks like determining this
will help. 

May be something like keep a track of number of processes on "sync-noidle"
tree and average read times when sync-noidle tree is being served. Over a
period of time we need to monitor what's the number of processes
(threshold), after which average read time goes up. For sync-noidle we can
then drive "queue_depth=nr_thrshold" and once queue depth reaches that,
then idle on the process. So for single spindle, I guess tipping point
will be 2 processes and we can idle on sync-noidle process. For more
spindles, tipping point will be higher.

These are just some random thoughts. 

> Until we can, we should optimize for the most common case.

Hard to say what's the common case? Single rotational disks or enterprise
storage with multiple disks behind RAID cards. 

> 
> Also, does the performance drop when the number of processes
> approaches 8*number of spindles?

I think here performance drop will be limited by queue depth. So once you
have more than 32 processes driving queue depth 32, it should not matter
how many processes you launch in parallel.

I have collected some numbers for running 1,2,4,8,32 and 64 threads in
parallel and see how throughput varies with vanilla kernel and with your
patch.

Vanilla kernel
==============
aggrb=2,771KB/s,
aggrb=2,779KB/s,
aggrb=3,084KB/s,
aggrb=3,623KB/s,
aggrb=3,847KB/s,
aggrb=3,940KB/s,
aggrb=4,216KB/s,

Patched kernel
==============
aggrb=2,778KB/s,
aggrb=2,447KB/s,
aggrb=2,240KB/s,
aggrb=2,182KB/s,
aggrb=2,082KB/s,
aggrb=2,033KB/s,
aggrb=1,672KB/s,

With vanilla kernel, output is on the rise as number of threads doing IO 
incrases and with patched kernel it is falling as number of threads
rise. This is not pretty.

Thanks
Vivek

> I think it should, so we will need to identify exacly the number of
> spindles to be able to
> allow only the right amount of parallelism.
> 
> > For yanmin's case, he seems to be running a case where there is only single
> > spindle and mulitple processes doing IO on that spindle. So I guess it
> > does not suffer from the fact that we are driving queue depth as 1.
> 
> Yes. Yanmin's configuration is a JBOD, i.e. a multi disk configuration
> in which no partition spans multiple disks, so you have at most 1 spindle
> per partition, possibly shared with others.
> 
> >
> > Why did I include some "deadline" numbers also? Just like that. Recently
> > I have also become interested in comaparing CFQ with "deadline" for
> > various workloads and see which one performs better in what
> > circumstances.
> >
> Sounds sensible, especially for RAID configurations it can be a good comparison.
> 
> Thanks,
> Corrado
> 
> > Thanks
> > Vivek
> >
> >> Reported-by: Yanmin Zhang <yanmin_zhang@...ux.intel.com>
> >> Signed-off-by: Corrado Zoccolo <czoccolo@...il.com>
> >> ---
> >>  block/cfq-iosched.c |   54 +++++++++++++-------------------------------------
> >>  1 files changed, 14 insertions(+), 40 deletions(-)
> >>
> >> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> >> index c6d5678..4e203c4 100644
> >> --- a/block/cfq-iosched.c
> >> +++ b/block/cfq-iosched.c
> >> @@ -133,9 +133,7 @@ struct cfq_queue {
> >>       unsigned short ioprio, org_ioprio;
> >>       unsigned short ioprio_class, org_ioprio_class;
> >>
> >> -     unsigned int seek_samples;
> >> -     u64 seek_total;
> >> -     sector_t seek_mean;
> >> +     u32 seek_history;
> >>       sector_t last_request_pos;
> >>       unsigned long seeky_start;
> >>
> >> @@ -1658,22 +1656,13 @@ static inline sector_t cfq_dist_from_last(struct cfq_data *cfqd,
> >>               return cfqd->last_position - blk_rq_pos(rq);
> >>  }
> >>
> >> -#define CFQQ_SEEK_THR                8 * 1024
> >> -#define CFQQ_SEEKY(cfqq)     ((cfqq)->seek_mean > CFQQ_SEEK_THR)
> >> +#define CFQQ_SEEK_THR                (sector_t)(8 * 100)
> >> +#define CFQQ_SEEKY(cfqq)     (hweight32(cfqq->seek_history) > 32/8)
> >>
> >>  static inline int cfq_rq_close(struct cfq_data *cfqd, struct cfq_queue *cfqq,
> >>                              struct request *rq, bool for_preempt)
> >>  {
> >> -     sector_t sdist = cfqq->seek_mean;
> >> -
> >> -     if (!sample_valid(cfqq->seek_samples))
> >> -             sdist = CFQQ_SEEK_THR;
> >> -
> >> -     /* if seek_mean is big, using it as close criteria is meaningless */
> >> -     if (sdist > CFQQ_SEEK_THR && !for_preempt)
> >> -             sdist = CFQQ_SEEK_THR;
> >> -
> >> -     return cfq_dist_from_last(cfqd, rq) <= sdist;
> >> +     return cfq_dist_from_last(cfqd, rq) <= CFQQ_SEEK_THR;
> >>  }
> >>
> >>  static struct cfq_queue *cfqq_close(struct cfq_data *cfqd,
> >> @@ -2971,30 +2960,16 @@ static void
> >>  cfq_update_io_seektime(struct cfq_data *cfqd, struct cfq_queue *cfqq,
> >>                      struct request *rq)
> >>  {
> >> -     sector_t sdist;
> >> -     u64 total;
> >> -
> >> -     if (!cfqq->last_request_pos)
> >> -             sdist = 0;
> >> -     else if (cfqq->last_request_pos < blk_rq_pos(rq))
> >> -             sdist = blk_rq_pos(rq) - cfqq->last_request_pos;
> >> -     else
> >> -             sdist = cfqq->last_request_pos - blk_rq_pos(rq);
> >> -
> >> -     /*
> >> -      * Don't allow the seek distance to get too large from the
> >> -      * odd fragment, pagein, etc
> >> -      */
> >> -     if (cfqq->seek_samples <= 60) /* second&third seek */
> >> -             sdist = min(sdist, (cfqq->seek_mean * 4) + 2*1024*1024);
> >> -     else
> >> -             sdist = min(sdist, (cfqq->seek_mean * 4) + 2*1024*64);
> >> +     sector_t sdist = 0;
> >> +     if (cfqq->last_request_pos) {
> >> +             if (cfqq->last_request_pos < blk_rq_pos(rq))
> >> +                     sdist = blk_rq_pos(rq) - cfqq->last_request_pos;
> >> +             else
> >> +                     sdist = cfqq->last_request_pos - blk_rq_pos(rq);
> >> +     }
> >>
> >> -     cfqq->seek_samples = (7*cfqq->seek_samples + 256) / 8;
> >> -     cfqq->seek_total = (7*cfqq->seek_total + (u64)256*sdist) / 8;
> >> -     total = cfqq->seek_total + (cfqq->seek_samples/2);
> >> -     do_div(total, cfqq->seek_samples);
> >> -     cfqq->seek_mean = (sector_t)total;
> >> +     cfqq->seek_history <<= 1;
> >> +     cfqq->seek_history |= (sdist > CFQQ_SEEK_THR);
> >>
> >>       /*
> >>        * If this cfqq is shared between multiple processes, check to
> >> @@ -3032,8 +3007,7 @@ cfq_update_idle_window(struct cfq_data *cfqd, struct cfq_queue *cfqq,
> >>               cfq_mark_cfqq_deep(cfqq);
> >>
> >>       if (!atomic_read(&cic->ioc->nr_tasks) || !cfqd->cfq_slice_idle ||
> >> -         (!cfq_cfqq_deep(cfqq) && sample_valid(cfqq->seek_samples)
> >> -          && CFQQ_SEEKY(cfqq)))
> >> +         (!cfq_cfqq_deep(cfqq) && CFQQ_SEEKY(cfqq)))
> >>               enable_idle = 0;
> >>       else if (sample_valid(cic->ttime_samples)) {
> >>               if (cic->ttime_mean > cfqd->cfq_slice_idle)
> >> --
> >> 1.6.4.4
> >
> 
> 
> 
> -- 
> __________________________________________________________________________
> 
> dott. Corrado Zoccolo                          mailto:czoccolo@...il.com
> PhD - Department of Computer Science - University of Pisa, Italy
> --------------------------------------------------------------------------
> The self-confidence of a warrior is not the self-confidence of the average
> man. The average man seeks certainty in the eyes of the onlooker and calls
> that self-confidence. The warrior seeks impeccability in his own eyes and
> calls that humbleness.
>                                Tales of Power - C. Castaneda
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