Re: [patch|rfc] add support for I/O scheduler tuning

[Jens Axboe <axboe@kernel.dk>]

On 2010-11-10 21:03, Vivek Goyal wrote:
> On Wed, Nov 10, 2010 at 01:26:21PM -0500, David Zeuthen wrote:
>> Hi,
>>
>> On Wed, Nov 10, 2010 at 11:47 AM, Jeff Moyer <jmoyer@redhat.com> wrote:
>>> Hi,
>>>
>>> From within the block layer in the kernel, it is difficult to
>>> automatically detect the performance characteristics of the underlying
>>> storage.  It was suggested by Jens Axboe at LSF2010 that we write a udev
>>> rule to tune the I/O scheduler properly for most cases.  The basic
>>> approach is to leave CFQ's default tunings alone for SATA disks.  For
>>> everything else, turn off slice idling and bump the quantum in order to
>>> drive higher queue depths.  This patch is an attempt to implement this.
>>>
>>> I've tested it in a variety of configurations:
>>> - cciss devices
>>> - sata disks
>>> - sata ssds
>>> - enterprise storage (single path)
>>> - enterprise storage (multi-path)
>>> - multiple paths to a sata disk (yes, you can actually do that!)
>>>
>>> The tuning works as expected in all of those scenarios.  I look forward
>>> to your comments.
>>
>> This looks useful, but I really think the kernel driver creating the
>> block device should choose/change the defaults for the created block
>> device - it seems really backwards to do this in user-space as an
>> afterthought.
> 
> I think it just becomes little easier to implement in user space so that
> if things don't work as expected, somebody can easily disable the rules
> or somebody can easily refine the rule further to better suite their
> needs instead of driver hardcoding this decision.

That's the primary reason why I suggested doing this in user space. Plus
we don't always know in the kernel, at least this provides an easier way
to auto-tune things.

-- 
Jens Axboe