Messages in this thread | | | From | Andi Kleen <> | Subject | Re: [PATCH] i386-pda UP optimization | Date | Tue, 21 Nov 2006 22:52:28 +0100 |
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> 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.
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