Re: [PATCH] cfq-iosched: rework seeky detection
From: Vivek Goyal
Date: Tue Jan 12 2010 - 14:13:15 EST
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.
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.
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.
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.
Thanks
Vivek
> Reported-by: Yanmin Zhang <yanmin_zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Corrado Zoccolo <czoccolo@xxxxxxxxx>
> ---
> 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
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