Re: vmalloc performance

[Minchan Kim <minchan.kim@gmail.com>]

On Mon, Apr 19, 2010 at 10:38 PM, Nick Piggin <npiggin@suse.de> wrote:
> On Mon, Apr 19, 2010 at 12:14:09AM +0900, Minchan Kim wrote:
>> On Fri, 2010-04-16 at 15:10 +0100, Steven Whitehouse wrote:
>> Nick, What do you think about "free area cache" approach?
>
> Thanks, yep something like this is what I had in mind. Looks like you
> have some really nice speed improvements which is great.
>
>
>> In this version, I don't consider last hole and backward cache movement which is
>> like mmap's cached_hole_size
>> That's because I want to flush vmap_areas freed intentionally if we meet vend.
>> It makes flush frequent than old but it's trade-off. In addition, vmalloc isn't
>> critical compared to mmap about performance. So I think that's enough.
>>
>> If you don't opposed, I will repost formal patch without code related to debug.
>
> I think I would prefer to be a little smarter about using lower
> addresses first. I know the lazy TLB flushing works against this, but
> that is an important speed tradeoff, wheras there is not really any
> downside to trying hard to allocate low areas first. Keeping virtual
> addresses dense helps with locality of reference of page tables, for
> one.
>
> So I would like to see:
> - invalidating the cache in the case of vstart being decreased.
> - Don't unconditionally reset the cache to the last vm area freed,
>  because you might have a higher area freed after a lower area. Only
>  reset if the freed area is lower.
> - Do keep a cached hole size, so smaller lookups can restart a full
>  search.

Firstly, I considered it which is used by mmap.
But I thought it might be overkill since vmalloc space isn't large
compared to mmaped addresses.
I should have thought about locality of reference of page tables. ;-)

> Probably also at this point, moving some of the rbtree code (like the
> search code) into functions would manage the alloc_vmap_area complexity.
> Maybe do this one first if you're going to write a patchset.
>
> What do you think? Care to have a go? :)

Good. I will add your requirements to TODO list.
But don't wait me. If you care to have a go, RUN!!!
I am looking forward to seeing your awesome patches. :)

Thanks for careful review, Nick.
-- 
Kind regards,
Minchan Kim

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