On Thu, 16 Sep 2010 08:48:25 +0200 (cest), Andi Kleen wrote:
When the dest and the src do overlap and the memory area is large, memmove
of
x86_64 is very inefficient, and it led to bad performance, such as btrfs's
file
deletion performance. This patch improved the performance of memmove on
x86_64
by using __memcpy_bwd() instead of byte copy when doing large memory area
copy
(len> 64).
I still don't understand why you don't simply use a backwards
string copy (with std) ? That should be much simpler and
hopefully be as optimized for kernel copies on recent CPUs.
But according to the comment of memcpy, some CPUs don't support "REP" instruction,
so I think we must implement a backwards string copy by other method for those CPUs,
But that implement is complex, so I write it as a function -- __memcpy_bwd().
Thanks!
Miao
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