linux-kernel - Re: [PATCH] blk-merge: fix blk_recount

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Date:	Tue, 14 Oct 2014 14:24:10 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Christoph Hellwig <hch@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kick In <pierre-andre.morey@...onical.com>,
	Chris J Arges <chris.j.arges@...onical.com>
Subject: Re: [PATCH] blk-merge: fix blk_recount_segments

Ming Lei <ming.lei@...onical.com> writes:
> Hi Rusty,

Hi Ming! 

        Sorry, I was on vacation.  I'm back and slowly working through
all my mail...

> 1, FIO script
> [global]
> direct=1
> size=128G
> bsrange=${BS}-${BS}
> timeout=60
> numjobs=4
> ioengine=libaio
> iodepth=64
> filename=/dev/vdb     #backed by /dev/nullb0, 4 virtqueues per virtio-blk
> group_reporting=1
>
> [f]
> rw=randread

> 5, result
> 5.1 without Rusty's virtio-vring patch
> - BS=4K, throughput: 179K
> - BS=256K, throughput: 27540

(ie. always using indirect)

> 5.2 with Rusty's virtio-vring patch
> - BS=4K, throughput: 173K
> - BS=256K, throughput: 25350

(ie. usually using indirect).

> Looks throughput decreases if BS is 256K in case of your patch.

Interesting.  Looks like we're ending up with fewer descs in flight,
though I'm surprised: with 256K blocks I'd expect us to hit the
steady state of "all indirect" almost immediately.

Hmm... I think the heuristic in my patch is flawed.  Let me try again,
and get back to you.

Thanks,
Rusty.
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