Re: [benchmark] 1% performance overhead of paravirt_ops on nativekernels

From: Ingo Molnar
Date: Tue Jun 09 2009 - 10:58:20 EST

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> I was benchmarking btrfs on my little EeePC. There, kmap overhead
> was 25% of file access time. Part of it is that people have been
> taught to use "kmap_atomic()", which is usable under spinlocks and
> people have been told that it's "fast". It's not fast. The whole
> TLB thing is slow as hell.

yeah. I noticed it some time ago that INVLPG is unreasonably slow.

My theory is that in the CPU it's perhaps a loop (in microcode?)
over _all_ TLBs - so as TLB caches get larger, INVLPG gets slower
and slower ...

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