Re: [PATCH] cfq-iosched: rework seeky detection

[Corrado Zoccolo]

On Mon, Jan 11, 2010 at 5:29 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> 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
>> Â the current head position, i.e. it is likely in the disk cache (disks
>> Â usually read more data than requested, and put it in cache for
>> Â subsequent reads). Otherwise, the request is classified as seeky.
>> * an history window of the last 32 requests is kept, storing the
>> Â classification result.
>> * A queue is marked as seeky if more than 1/8 of the last 32 requests
>> Â were seeky.
>>
>
> Because we are not relying on long term average and looking at only last
> 32 requests, looks like we will be switching between seeky to non seeky
> much more aggressively.
Hi Vivek,
I hope this is not the case. I remember you observed instability in your tests.
If you can re-run such tests, we can see if the instability is reduced
or increased.

>
>> This patch fixes a regression reported by Yanmin, on mmap 64k random
>> reads.
>
> We never changed the seek logic recently. So if it is a regression it must
> have been introduced by some other change and we should look and fix that
> too.
We didn't change the seek logic, but changed other code that exposed
the bug in it.

The cause of regression is that, in 2.6.32, with low_latency=1, even queues that
were marked as seeky still had idle slice (so the metric didn't
count), while now
we are jumping immediately to a new queue.
So, due to our removing the idle, we have to fix the bug in the metric
(exposed by my other code change), that caused this regression,
by making the seeky detection better.

BTW, the issue only shows with really a lot of processes doing I/O, so
that the disk
cache is reclaimed before the new request comes.

>
> That's a different thing that seeky queue detection logic change also gave
> performance improvement in this specific case.
>
> IIUC, you are saying that doing bigger block size IO on mmapped files, has
> few 4K requests one after the other and then a big seek. So in such cases
> you would rather mark the cfqq as sync-idle and idle on the queue so that
> we can server 16 (64K/4) requests soon and then incur a large seek time.
Yes. Assuming a seq request takes 1ms (worst case for external USB disks)
and a random one 8ms, 15 seq requests and 1 seek complete in 23 ms,
that is just 43%
more than the completion of 16 seq requests.

The threshold is defined as the point at which roughly 50% of the disk bandwidth
is achieved. This doesn't consider think time, though.

>
> So these requests strictly come one after the other and request merging takes
> place?
Request merging doesn't take place, since with mmap, the read is triggered by a
page fault, so the process is waiting for the first request to be
completed before
issuing the next one.

Thanks,
Corrado
>
>>
>> Reported-by: Yanmin Zhang <yanmin_zhang@xxxxxxxxxxxxxxx>
>> Signed-off-by: Corrado Zoccolo <czoccolo@xxxxxxxxx>
>> ---
>> Âblock/cfq-iosched.c | Â 54 +++++++++++++-------------------------------------
>> Â1 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 {
>> Â Â Â unsigned short ioprio, org_ioprio;
>> Â Â Â unsigned short ioprio_class, org_ioprio_class;
>>
>> - Â Â unsigned int seek_samples;
>> - Â Â u64 seek_total;
>> - Â Â sector_t seek_mean;
>> + Â Â u32 seek_history;
>> Â Â Â sector_t last_request_pos;
>> Â Â Â unsigned long seeky_start;
>>
>> @@ -1658,22 +1656,13 @@ static inline sector_t cfq_dist_from_last(struct cfq_data *cfqd,
>> Â Â Â Â Â Â Â return cfqd->last_position - blk_rq_pos(rq);
>> Â}
>>
>> -#define CFQQ_SEEK_THR Â Â Â Â Â Â Â Â8 * 1024
>> -#define CFQQ_SEEKY(cfqq) Â Â ((cfqq)->seek_mean > CFQQ_SEEK_THR)
>> +#define CFQQ_SEEK_THR Â Â Â Â Â Â Â Â(sector_t)(8 * 100)
>
> What's the rational behind changing CFQQ_SEEK_THR from 8*1024 to 8*100?
>
> Vivek
>
>> +#define CFQQ_SEEKY(cfqq) Â Â (hweight32(cfqq->seek_history) > 32/8)
>>
>> Âstatic inline int cfq_rq_close(struct cfq_data *cfqd, struct cfq_queue *cfqq,
>> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Âstruct request *rq, bool for_preempt)
>> Â{
>> - Â Â sector_t sdist = cfqq->seek_mean;
>> -
>> - Â Â if (!sample_valid(cfqq->seek_samples))
>> - Â Â Â Â Â Â sdist = CFQQ_SEEK_THR;
>> -
>> - Â Â /* if seek_mean is big, using it as close criteria is meaningless */
>> - Â Â if (sdist > CFQQ_SEEK_THR && !for_preempt)
>> - Â Â Â Â Â Â sdist = CFQQ_SEEK_THR;
>> -
>> - Â Â return cfq_dist_from_last(cfqd, rq) <= sdist;
>> + Â Â return cfq_dist_from_last(cfqd, rq) <= CFQQ_SEEK_THR;
>> Â}
>>
>> Âstatic struct cfq_queue *cfqq_close(struct cfq_data *cfqd,
>> @@ -2971,30 +2960,16 @@ static void
>> Âcfq_update_io_seektime(struct cfq_data *cfqd, struct cfq_queue *cfqq,
>> Â Â Â Â Â Â Â Â Â Â Âstruct request *rq)
>> Â{
>> - Â Â sector_t sdist;
>> - Â Â u64 total;
>> -
>> - Â Â if (!cfqq->last_request_pos)
>> - Â Â Â Â Â Â sdist = 0;
>> - Â Â else if (cfqq->last_request_pos < blk_rq_pos(rq))
>> - Â Â Â Â Â Â sdist = blk_rq_pos(rq) - cfqq->last_request_pos;
>> - Â Â else
>> - Â Â Â Â Â Â sdist = cfqq->last_request_pos - blk_rq_pos(rq);
>> -
>> - Â Â /*
>> - Â Â Â* Don't allow the seek distance to get too large from the
>> - Â Â Â* odd fragment, pagein, etc
>> - Â Â Â*/
>> - Â Â if (cfqq->seek_samples <= 60) /* second&third seek */
>> - Â Â Â Â Â Â sdist = min(sdist, (cfqq->seek_mean * 4) + 2*1024*1024);
>> - Â Â else
>> - Â Â Â Â Â Â sdist = min(sdist, (cfqq->seek_mean * 4) + 2*1024*64);
>> + Â Â sector_t sdist = 0;
>> + Â Â if (cfqq->last_request_pos) {
>> + Â Â Â Â Â Â if (cfqq->last_request_pos < blk_rq_pos(rq))
>> + Â Â Â Â Â Â Â Â Â Â sdist = blk_rq_pos(rq) - cfqq->last_request_pos;
>> + Â Â Â Â Â Â else
>> + Â Â Â Â Â Â Â Â Â Â sdist = cfqq->last_request_pos - blk_rq_pos(rq);
>> + Â Â }
>>
>> - Â Â cfqq->seek_samples = (7*cfqq->seek_samples + 256) / 8;
>> - Â Â cfqq->seek_total = (7*cfqq->seek_total + (u64)256*sdist) / 8;
>> - Â Â total = cfqq->seek_total + (cfqq->seek_samples/2);
>> - Â Â do_div(total, cfqq->seek_samples);
>> - Â Â cfqq->seek_mean = (sector_t)total;
>> + Â Â cfqq->seek_history <<= 1;
>> + Â Â cfqq->seek_history |= (sdist > CFQQ_SEEK_THR);
>>
>> Â Â Â /*
>> Â Â Â Â* 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,
>> Â Â Â Â Â Â Â cfq_mark_cfqq_deep(cfqq);
>>
>> Â Â Â if (!atomic_read(&cic->ioc->nr_tasks) || !cfqd->cfq_slice_idle ||
>> - Â Â Â Â (!cfq_cfqq_deep(cfqq) && sample_valid(cfqq->seek_samples)
>> - Â Â Â Â Â&& CFQQ_SEEKY(cfqq)))
>> + Â Â Â Â (!cfq_cfqq_deep(cfqq) && CFQQ_SEEKY(cfqq)))
>> Â Â Â Â Â Â Â enable_idle = 0;
>> Â Â Â else if (sample_valid(cic->ttime_samples)) {
>> Â Â Â Â Â Â Â if (cic->ttime_mean > cfqd->cfq_slice_idle)
>> --
>> 1.6.4.4
>



-- 
__________________________________________________________________________

dott. Corrado Zoccolo                          mailto:czoccolo@xxxxxxxxx
PhD - Department of Computer Science - University of Pisa, Italy
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