Re: [PATCH] i386-pda UP optimization

[Jeremy Fitzhardinge <jeremy@goop.org>]

Eric Dumazet wrote:
> for umask/getppid(), its a basic loop with 100.000.000 iterations

Ah, OK, so there's about 2.5-3.5ns difference due to the instructions
you removed.  That's very much in line with that I saw in my measurements.

> for read/write(), loop with 10.000.000 iterations

2 syscalls/iteration?  It's interesting you measured about the same
absolute time difference (.42s) even though you're doing 1/5th the
number of syscalls.

> elapsed time (/usr/bin/time ./prog)
> 10 runs, and the minimum time is taken.

Hm, but "time" measures user, system and real time.  You used real time?

> Hum... Do you mean a cache miss every time we do a syscall ? What
> could invalidate this cache exactly ?

Well, there might be a miss simply because the line got evicted.  But as
Andi pointed out, a hot benchmark like this is very unlikely to get any
cache misses unless there's something very unfortunate happening.

    J