> 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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