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

[Avi Kivity <avi@redhat.com>]

Ingo Molnar wrote:
> * Linus Torvalds <torvalds@linux-foundation.org> 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 ...
>   

The tlb already content-addresses entries when looking up translations, 
so it shouldn't be that bad.

invlpg does have to invalidate all the intermediate entries 
("paging-structure caches"), and it does (obviously) force a tlb reload.

I seem to recall 50 cycles for invlpg, what do you characterize as 
unreasonably slow?

-- 
error compiling committee.c: too many arguments to function