lkml.org 
[lkml]   [2018]   [Nov]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRE: [PATCH] x86: only use ERMS for user copies for larger sizes
Date
From: Denys Vlasenko
> Sent: 21 November 2018 13:44
...
> I also tested this while working for string ops code in musl.
>
> I think at least 128 bytes would be the minimum where "REP insn"
> are more efficient. In my testing, it's more like 256 bytes...

What happens for misaligned copies?
I had a feeling that the ERMS 'reb movsb' code used some kind
of barrel shifter in that case.

The other problem with the ERMS copy is that it gets used
for copy_to/from_io() - and the 'rep movsb' on uncached
locations has to do byte copies.
Byte reads on PCIe are really horrid.

David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
\
 
 \ /
  Last update: 2018-11-22 18:36    [W:0.140 / U:1.120 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site