linux-kernel - Re: Flash IO slow 1.5 MB/s

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Date:	Tue, 4 May 2010 12:00:46 -0600
From:	"Trenton D. Adams" <trenton.d.adams@...il.com>
To:	Paul Hartman <paul.hartman+linux@...il.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Flash IO slow 1.5 MB/s

On Tue, May 4, 2010 at 11:34 AM, Paul Hartman
<paul.hartman+linux@...il.com> wrote:
> On Mon, May 3, 2010 at 10:52 PM, Trenton D. Adams
> <trenton.d.adams@...il.com> wrote:
>> It really looks like there's a scheduling issue.  It seems as if the
>> system is IO thrashing on the flash drive, and bounces all over the
>> place in terms of performance.  Sometimes it's really low, like the
>> 2.73M/s, and other times it's really fast, like the 28.86M/s.
>> Although you can't see it there, there were times when rsync was
>> registering 200kb/s.  None of them are "really" accurate, as
>> everything is queued for writing, but the final results of 1.5M/s
>> (calculated from the "real" time) is terrible.
>
> I have a similar experience (posted to this list a few months ago)
> with mounting a flash device (mobile phone) in USB mass storage mode.
> When I/O scheduler for that device is CFQ, write performance is really
> terrible. When I change the scheduler to deadline, performance is
> several times better. In 2.6.32 pdflush was replaced and CFQ
> performance saw a 4x increase but still far too slow.
>
> CFQ in <=2.6.31: 450KB/sec
> CFQ in >=2.6.32: 2MB/sec
> Deadline in all: 17MB/sec
>
> I didn't try anything with dirty_bytes.
>
> FWIW :)

Oops, my message didn't reach the LKML, sorry for the spam Paul.

I switched to deadline and dirty_ratio 20 for my flash device, and I
am seeing VERY slow performance as well.  I get a lot of freezing up
of rsync, where the progress just stops (visually anyhow), which is
the same as what I see with cfq.  However, it's not 14 minutes as it
was in my original email...

[11:44 trenta@...notebook web] $ time rsync -v --progress
/home/share/DVD/*.avi /media/disk/
facing-the-giants.avi
  709911016 100%    5.49MB/s    0:02:03 (xfer#1, to-check=1/2)
jonah.avi
  621254748 100%   15.97MB/s    0:00:37 (xfer#2, to-check=0/2)

sent 1331328404 bytes  received 50 bytes  4430377.55 bytes/sec
total size is 1331165764  speedup is 1.00

real    4m59.657s
user    0m8.553s
sys     0m9.501s


with dirty_bytes 16000000, I still get twice the speed out of deadline.

[11:53 trenta@...notebook web] $ time rsync -v --progress
/home/share/DVD/*.avi /media/disk/
facing-the-giants.avi
  709911016 100%    7.62MB/s    0:01:28 (xfer#1, to-check=1/2)
jonah.avi
  621254748 100%    7.64MB/s    0:01:17 (xfer#2, to-check=0/2)

sent 1331328404 bytes  received 50 bytes  7948229.58 bytes/sec
total size is 1331165764  speedup is 1.00

real    2m47.244s
user    0m8.429s
sys     0m9.377s


So, perhaps it's a combination of the schedulers and something else in
the kernel?  And perhaps, CFQ just amplifies something else in the
kernel, more than deadline does?
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