linux-kernel - Re: Big I/O latencies, except when iotop is hooked

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Date:	Thu, 10 May 2012 10:24:28 +0200
From:	Felipe Contreras <felipe.contreras@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Big I/O latencies, except when iotop is hooked

On Wed, May 9, 2012 at 7:56 PM, Arnd Bergmann <arnd@...db.de> wrote:

> The first thing you should check is whether the partitions are
> properly aligned, using 'fdisk -l -u /dev/sdX'.

I aligned it to 4MiB, it doesn't make much of a difference.

> Of course none of this will tell you why iotop makes any difference.

After trying a bit more I can still see the problem with iotop, I
could swear it happens much less often, but I can't really be sure
now.

I tried with different io schedulers, and it seems cfq does better, I
also tried different priorities, but the issue was still there.

I also tried to setup the crypto partition with align-payload, to no
avail, and to get rid of crypto altogether. I also tried with multiple
ext4 format options, but they seemed to make matters worst.

I finally tried on a normal disk device, and it works perfectly there
(even with encryption); so the issue happens only on SSD/usb key.

Cheers.

-- 
Felipe Contreras
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