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

[Bob Liu]

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.

> default amount of persistent grants blkback can handle, so the LRU in
> blkback will kick in.
> 

Sounds good.

-- 
Regards,
-Bob
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