linux-kernel - Re: [PATCH 04/10] xen/blkfront: separate ring information to an new struct

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Message-ID: <5508CC61.60809@oracle.com>
Date:	Wed, 18 Mar 2015 08:52:49 +0800
From:	Bob Liu <bob.liu@...cle.com>
To:	Felipe Franciosi <felipe.franciosi@...rix.com>
CC:	Malcolm Crossley <malcolm.crossley@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Roger Pau Monne <roger.pau@...rix.com>,
	David Vrabel <david.vrabel@...rix.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"axboe@...com" <axboe@...com>,
	"hch@...radead.org" <hch@...radead.org>,
	"avanzini.arianna@...il.com" <avanzini.arianna@...il.com>,
	"chegger@...zon.de" <chegger@...zon.de>
Subject: Re: [PATCH 04/10] xen/blkfront: separate ring information to an new
 struct


On 03/17/2015 10:52 PM, Felipe Franciosi wrote:
> Hi Bob,
> 
> I've put the hardware back together and am sorting out the software for testing. Things are not moving as fast as I wanted due to other commitments. I'll keep this thread updated as I progress. Malcolm is OOO and I'm trying to get his patches to work on a newer Xen.
> 

Thank you!

> The evaluation will compare:
> 1) bare metal i/o (for baseline)
> 2) tapdisk3 (currently using grant copy, which is what scales best in my experience)
> 3) blkback w/ persistent grants
> 4) blkback w/o persistent grants (I will just comment out the handshake bits in blkback/blkfront)
> 5) blkback w/o persistent grants + Malcolm's grant map patches
> 

I think you need to add the patches from Christoph Egger with title
"[PATCH v5 0/2] gnttab: Improve scaleability" here.
http://lists.xen.org/archives/html/xen-devel/2015-02/msg01188.html


> To my knowledge, blkback (w/ or w/o persistent grants) is always faster than user space alternatives (e.g. tapdisk, qemu-qdisk) as latency is much lower. However, tapdisk with grant copy has been shown to produce (much) better aggregate throughput figures as it avoids any issues with grant (un)mapping.
> 
> I'm hoping to show that (5) above scales better than (3) and (4) in a representative scenario. If it does, I will recommend that we get rid of persistent grants in favour of a better and more scalable grant (un)mapping implementation.
> 

Right, but even if 5) have better performance, we have to make sure
older hypervisors with new linux kernel won't be affected after get rid
of persistent grants.

-- 
Regards,
-Bob
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