Re:

Brandon Casey wrote:

Linus Torvalds wrote:

quoted

And 
the preloading sounds like it hits serialization overhead in the kernel, 
which I'm not at all surprised at, but not being surprised doesn't mean 
that I'm not interested to hear where it is.

The Linux VFS dcache itself should scale better than that (but who knows - 
cacheline ping-pong due to lock contention can easily cause a 10x slowdown 
even without being _totally_ contended all the time). So I would _suspect_ 
that it's some NFS lock that you're seeing, but I'd love to know more.

Btw, those system times are pretty high to begin with, so I'd love to know 
kernel version and see a profile even without the parallel case and 
presumably lock contention.

Here's an strace of 'git checkout':

Before (cold cache):
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 98.60    6.365501         111     57432           lstat64
  0.50    0.031984         359        89         2 close
  0.25    0.015818         115       137        77 open
  0.12    0.007670          23       339           write
  0.09    0.005631         110        51           munmap
  0.08    0.004873          49        99        69 stat64
  0.07    0.004771         140        34        15 access
  0.05    0.003083         280        11         5 waitpid
  0.05    0.002973          10       284           brk
  0.04    0.002816         469         6           execve
<snip>

After (cold cache, no lstat fix, just cache_preload):
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 90.90   23.717981         413     57432           lstat64
  8.72    2.273917      162423        14         2 futex
  0.12    0.032241         948        34           close
  0.04    0.011507         202        57           munmap
  0.04    0.009648         132        73           mmap2
  0.03    0.008508         149        57        20 open
  0.03    0.007771         311        25           mprotect
  0.03    0.007758         388        20           clone
  0.03    0.007548          23       334           write
  0.02    0.005247         262        20        10 access

-brandon