linux-kernel - Re: [PATCH 5/5] virtio-blk: implement ->make

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Message-ID: <20111006134257.GB20190@infradead.org>
Date:	Thu, 6 Oct 2011 09:42:57 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Jens Axboe <axboe@...nel.dk>,
	Stefan Hajnoczi <stefanha@...ux.vnet.ibm.com>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] virtio-blk: implement ->make_request

On Thu, Oct 06, 2011 at 12:22:14PM +1030, Rusty Russell wrote:
> On Wed, 05 Oct 2011 15:54:08 -0400, Christoph Hellwig <hch@...radead.org> wrote:
> > Add an alternate I/O path that implements ->make_request for virtio-blk.
> > This is required for high IOPs devices which get slowed down to 1/5th of
> > the native speed by all the locking, memory allocation and other overhead
> > in the request based I/O path.
> 
> Ouch.
> 
> I'd be tempted to just switch across to this, though I'd be interested
> to see if the simple add_buf change I referred to before has some effect
> by itself (I doubt it).

Benchmarking this more extensively even on low-end devices is number
on my todo list after sorting out the virtqueue race and implementing
flush/fua support.  I'd really prefer to switch over to it
unconditionally if the performance numbers allow it.

> Also, though it's overkill I'd use standard list primitives rather than
> open-coding a single linked list.

I really prefer using standard helpers, but using a doubly linked list
and increasing memory usage seems like such a waste.  Maybe I should
annoy Linus by proposing another iteration of a common single linked
list implementation :)

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