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SubjectRe: Bad SSD performance with recent kernels
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Le lundi 30 janvier 2012 à 22:28 +0800, Wu Fengguang a écrit :
> On Mon, Jan 30, 2012 at 06:31:34PM +0800, Li, Shaohua wrote:
>
> > Looks the 2.6.39 block plug introduces some latency here. deleting
> > blk_start_plug/blk_finish_plug in generic_file_aio_read seems
> > workaround
> > the issue. The plug seems not good for sequential IO, because readahead
> > code already has plug and has fine grained control.
>
> Why not remove the generic_file_aio_read() plug completely? It
> actually prevents unplugging immediately after the readahead IO is
> submitted and in turn stalls the IO pipeline as showed by Eric's
> blktrace data.
>
> Eric, will you test this patch? Thank you.
>
> --- linux.orig/mm/filemap.c 2012-01-30 22:27:11.000000000 +0800
> +++ linux/mm/filemap.c 2012-01-30 22:27:25.000000000 +0800
> @@ -1414,15 +1414,12 @@ generic_file_aio_read(struct kiocb *iocb
> unsigned long seg = 0;
> size_t count;
> loff_t *ppos = &iocb->ki_pos;
> - struct blk_plug plug;
>
> count = 0;
> retval = generic_segment_checks(iov, &nr_segs, &count, VERIFY_WRITE);
> if (retval)
> return retval;
>
> - blk_start_plug(&plug);
> -
> /* coalesce the iovecs and go direct-to-BIO for O_DIRECT */
> if (filp->f_flags & O_DIRECT) {
> loff_t size;
> @@ -1495,7 +1492,6 @@ generic_file_aio_read(struct kiocb *iocb
> break;
> }
> out:
> - blk_finish_plug(&plug);
> return retval;
> }
> EXPORT_SYMBOL(generic_file_aio_read);


Excellent results

# echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sdb of=/dev/null bs=2M count=2048
2048+0 enregistrements lus
2048+0 enregistrements écrits
4294967296 octets (4,3 GB) copiés, 16,2309 s, 265 MB/s

# echo 3 >/proc/sys/vm/drop_caches ;dd if=/dev/sdb of=/dev/null bs=128k count=32768
32768+0 enregistrements lus
32768+0 enregistrements écrits
4294967296 octets (4,3 GB) copiés, 16,3153 s, 263 MB/s

# hdparm -t /dev/sdb

/dev/sdb:
Timing buffered disk reads: 752 MB in 3.00 seconds = 250.62 MB/sec


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