memcpy vs memmove

[Dirk Geschke <dirk@lug-erding.de>]

Hi glibc maintainers,

I have a performance "problem" which I do not understand. If I allocate
a memory buffer of 1 GB (aligned to pagesize) and do a memset(), then
this takes 1.1 seconds. If I copy this buffer to another one using
memcpy(), it takes about 1.7 seconds. But if I use memmove() or bcopy()
it takes only 0.7s and 0.5s:

  memset  took 1.100437 seconds -> 930.54 MB/s
  memcpy  took 1.366200 seconds -> 749.52 MB/s 
  bcopy   took 0.703371 seconds -> 1.42 GB/s 
  memmove took 0.545014 seconds -> 1.83 GB/s 

I tried to look at the source code, but got lost somewhere. For me it
looks like they are all using the same code. So I do not understand why
memcpy() is so slow. Do you have any ideas, hints, pointers, ...?

>From the C source it looks identically, but there are also some
assembler codes in the source tree and finally I am not sure which
code is used for which function.

I am using wheezy,

  libc6:amd64    2.13-35      amd64

but the squeeze version shows the same behaviour. Maybe there is some
room to optimize memcpy()?

Best regards

Dirk

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