linux-kernel - Re: [PATCH 6/6] writeback: throttle buffered writeback

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Message-ID: <56F1A8D0.4060403@fb.com>
Date:	Tue, 22 Mar 2016 14:19:28 -0600
From:	Jens Axboe <axboe@...com>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	<linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
	<linux-block@...r.kernel.org>
Subject: Re: [PATCH 6/6] writeback: throttle buffered writeback

On 03/22/2016 02:12 PM, Jeff Moyer wrote:
> Hi, Jens,
>
> Jens Axboe <axboe@...com> writes:
>
>> If the device has write back caching, 'wb_cache_delay' delays by
>> this amount of usecs when a write completes before allowing more.
>
> What's the reason behind that?

For classic write back caching, the cache can absorb a bunch of writes 
shortly, which means that the completion cost only shows a small part of 
the overall cost. This means that if we just throttle on completion, 
then when the device starts committing to media, then we'll end up 
starving other IO anyway. This knob is a way to attempt to tame that.

-- 
Jens Axboe