linux-kernel - Re: [PATCH v2 1/1] mmc: block: Add write packing control

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Date:	Mon, 11 Jun 2012 22:49:53 +0530
From:	"S, Venkatraman" <svenkatr@...com>
To:	merez@...eaurora.org
Cc:	Seungwon Jeon <tgih.jun@...sung.com>, linux-mmc@...r.kernel.org,
	linux-arm-msm@...r.kernel.org,
	"DOCUMENTATION'" <linux-doc@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/1] mmc: block: Add write packing control

On Sat, Jun 9, 2012 at 8:16 PM,  <merez@...eaurora.org> wrote:
>
>> Hi,
>>
>> How can we check the effect?
>> Do you have any result?
> We ran parallel lmdd read and write operations and found out that the
> write packing causes the read throughput to drop from 24MB/s to 12MB/s.

Whoa! That's a big drop.
BTW, is there a problem with throughput or latency, or both ?
If these numbers are over long duration (>5 seconds), then where are
the cycles going?
It would be nice to see some blktrace figures for the issue, and then fix it,
rather than apply a band aid like the write-packing-control on top..


> The write packing control managed to increase the read throughput back to
> the original value.
> We also examined "real life" scenarios, such as performing a big push
> operation in parallel to launching several applications. We measured the
> read latency and found out that with the write packing control the worst
> case of the read latency was smaller.
>
>> Please check the several comment below.
>>
>> Maya Erez <merez@...eaurora.org> wrote:
>>> The write packing control will ensure that read requests latency is
>>> not increased due to long write packed commands.
>>>
>>> The trigger for enabling the write packing is managing to pack several
>>> write requests. The number of potential packed requests that will
>>> trigger
>>> the packing can be configured via sysfs by writing the required value
>>> to:
>>> /sys/block/<block_dev_name>/num_wr_reqs_to_start_packing.
>>> The trigger for disabling the write packing is fetching a read request.
>>>
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