Re: [RFC 0/8] Reliably generate large request from SRP

Bart Van Assche
2011-01-24T16:14:00Z

On Mon, Jan 24, 2011 at 4:32 PM, Or Gerlitz <ogerlitz-smomgflXvOZWk0Htik3J/w@public.gmane.org> wrote:
> David Dillow wrote:
>>> if we look on the 50% for SAS/1M IOs that you're presenting, can you tell
>>> what made the difference, srp went from sg_tablesize of 255 to 256 so the
>>> upper layers where able to provide 1M as one IO
>
>> This win is from sg_tablesize going from 255 to 256 in this case; the HW
>> really likes that better than getting two requests -- one for 1020 KB
>> and one for 4 KB.
>
> Its always nice to find the simplest explanation to the greatest improvement... going to the 2nd largest gains
>
>> SAS   2M      520 MB/s        861 MB/s
>> SAS   4M      529 MB/s        921 MB/s
>> SAS   8M      600 MB/s        951 MB/s
>
> I wonder what made the difference here? it can't be only the 255 --> 256 sg_tablesize change, for the 2M case
> the change to use 512 pages FMRs could let you use one rkey/fmr for the whole IO but not for 4M/8M

I think it would be interesting to have performance measurements with
a RAM disk as target too because it is hard to tell for someone not
familiar with the internals of the target used in this test which
performance gain is due to the initiator changes and which is due to
the target behavior.

Bart.