Subject: Re: [PATCH] blk-flush: Queue through IO scheduler when flush not
 required
To: Jan Kara <jack@suse.com>
References: <1447334752-31027-1-git-send-email-jack@suse.com>
Cc: Jeff Moyer <jmoyer@redhat.com>, LKML <linux-kernel@vger.kernel.org>
From: Jens Axboe <axboe@kernel.dk>
Message-ID: <564A58A3.6020005@kernel.dk>
Date: Mon, 16 Nov 2015 15:28:51 -0700
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On 11/12/2015 06:25 AM, Jan Kara wrote:
> Currently blk_insert_flush() just adds flush request to q->queue_head
> when flush is not required. That completely bypasses IO scheduler so
> e.g. CFQ can be idling waiting for new request to arrive and will idle
> through the whole window unnecessarily. Luckily this only happens in
> rare cases as usually checks in generic_make_request_checks() clear
> FLUSH and FUA flags early if they are not needed.
>
> When no flushing is actually required, we can easily fix the problem by
> properly queueing the request through the IO scheduler. Ideally IO
> scheduler should be also made aware of requests queued via
> blk_flush_queue_rq(). However inserting flush request through IO
> scheduler can have unwanted side-effects since due to flush batching
> delaying the flush request in IO scheduler will delay all flush requests
> possibly coming from other processes. So we keep adding the request
> directly to q->queue_head.

Applied, thanks Jan.

-- 
Jens Axboe

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