Re: [PATCH] i386-pda UP optimization

[Andi Kleen <ak@suse.de>]

> For umask/getppid, assuming you're just running 1e7 iterations, you're
> seeing a difference of 25 and 35ns per iteration difference.  I wonder
> why it would be different for different syscalls; I would expect it to
> be a constant overhead either way.

They got different numbers of current references? 

> Certainly these numbers are much 
> larger than I saw when I benchmarked pda-vs-nopda using lmbench's null
> syscall (getppid) test; I saw an overall 9ns difference in null syscall
> time on my Core Duo run at 1GHz.  What's your CPU and speed?
> 
> One possibility is a cache miss on the gdt while reloading %gs.  I've

On such micro benchmarks everything should be cache hot in theory
(unless it's a system with really small cache)

> been planning on a patch to rearrange the gdt in order to pack all the
> commonly used segment descriptors into one or two cache lines so that
> all the segment register reloads can be done with a minimum of cache
> misses.  It would be interesting for you to replace the:
> 
>     movl $(__KERNEL_PDA), %edx; movl %edx, %gs
> 
> with an appropriate read of the gdt entry, hm, which is a bit complex to
> find.

On UP it could be hardcoded. And oprofile can be used to profile for cache misses.

-Andi
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