linux-kernel - Re: Bad SSD performance with recent kernels

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Date:	Mon, 30 Jan 2012 15:51:49 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Wu Fengguang <wfg@...ux.intel.com>
Cc:	Shaohua Li <shaohua.li@...el.com>,
	Herbert Poetzl <herbert@...hfloor.at>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>
Subject: Re: Bad SSD performance with recent kernels

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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