Re: [Xen-devel] [PATCH] xen/blkfront: increase the default value of xen_blkif_max_segments

[David Vrabel <david.vrabel@citrix.com>]

On 17/12/14 16:13, Konrad Rzeszutek Wilk wrote:
> On Wed, Dec 17, 2014 at 04:18:58PM +0800, Bob Liu wrote:
>>
>> On 12/16/2014 06:32 PM, Roger Pau Monné wrote:
>>> El 16/12/14 a les 11.11, Bob Liu ha escrit:
>>>> The default maximum value of segments in indirect requests was 32, IO
>>>> operations with bigger block size(>32*4k) would be split and performance start
>>>> to drop.
>>>>
>>>> Nowadays backend device usually support 512k max_sectors_kb on desktop, and
>>>> may larger on server machines with high-end storage system.
>>>> The default size 128k was not very appropriate, this patch increases the default
>>>> maximum value to 128(128*4k=512k).
>>>
>>> This looks fine, do you have any data/graphs to backup your reasoning?
>>>
>>
>> I only have some results for 1M block size FIO test but I think that's
>> enough.
>>
>> xen_blkfront.max 	Rate (MB/s) 	Percent of Dom-0
>> 32 	11.1 	31.0%
>> 48 	15.3 	42.7%
>> 64 	19.8 	55.3%
>> 80 	19.9 	55.6%
>> 96 	23.0 	64.2%
>> 112 	23.7 	66.2%
>> 128 	31.6 	88.3%
>>
>> The rates above are compared against the dom-0 rate of 35.8 MB/s.
>>
>>> I would also add to the commit message that this change implies we can
>>> now have 32*128+32 = 4128 in-flight grants, which greatly surpasses the
>>
>> The number could be larger if using more pages as the
>> xen-blkfront/backend ring based on Wei Liu's patch "xenbus_client:
>> extend interface to suppurt multi-page ring", it helped improve the IO
>> performance a lot on our system connected with high-end storage.
>> I'm preparing resend related patches.
> 
> Or potentially making the request and response be seperate rings - and the
> response ring entries not tied in to the request. As in right now if we
> have an request at say slot 1,5, and 7, we expect the response to be at
> slot 1,5, and 7 as well.

No. Responses are placed in the first available slot.  The response is
associated with the original request by the ID field.

See make_response().

David