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To: Jens Axboe <axboe@suse.de>
Cc: Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] deadline io scheduler
References: <20020925172024.GH15479@suse.de> <3D92A61E.40BFF2D0@digeo.com>
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In-Reply-To: <20020926082925.GK12862@suse.de> (Jens Axboe's message of "Thu,
 26 Sep 2002 10:29:25 +0200")
From: Daniel Pittman <daniel@rimspace.net>
Organization: Not today, thank you, Mother.
Date: Fri, 27 Sep 2002 09:23:39 +1000
Message-ID: <873crw5o90.fsf@enki.rimspace.net>
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On Thu, 26 Sep 2002, Jens Axboe wrote:
> On Thu, Sep 26 2002, Daniel Pittman wrote:
>> On Thu, 26 Sep 2002, Jens Axboe wrote:
>> > On Wed, Sep 25 2002, Andrew Morton wrote:
>> 
>> [...]
>> 
>> > writes_starved. This controls how many times reads get preferred
>> > over writes. The default is 2, which means that we can serve two
>> > batches of reads over one write batch. A value of 4 would mean that
>> > reads could skip ahead of writes 4 times. A value of 1 would give
>> > you 1:1 read:write, ie no read preference. A silly value of 0 would
>> > give you write preference, always.
>> 
>> Actually, a value of zero doesn't sound completely silly to me, right
>> now, since I have been doing a lot of thinking about video capture
>> recently.
>> 
>> How much is it going to hurt a filesystem like ext[23] if that value
>> is set to zero while doing large streaming writes -- something like
>> (almost) uncompressed video at ten to twenty meg a second, for
>> gigabytes?
> 
> You are going to stalll all reads indefinately :-)

Which has some potentially fatal consequences, really, if any of the
capture code gets paged out before the streaming write starts, or if the
filesystem needs to read a bitmap block or so, as Rik points out.

>> This is a situation where, for a dedicated machine, delaying reads
>> almost forever is actually a valuable thing. At least, valuable until
>> it stops the writes from being able to proceed.
> 
> Well 0 should achieve that quite fine

Would you consider allowing something akin to 'writes_starved = -4' to
allow writes to bypass reads only 4 times -- a preference for writes,
but not forever?

That's going to express the bias I (think I) want for this case, but
it's not going to be able to stall a read forever...

     Daniel

-- 
It is quite humbling to realize that the storage occupied by the longest line
from a typical Usenet posting is sufficient to provide a state space so vast
that all the computation power in the world can not conquer it.
        -- Dave Wallace
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