Re: missing madvise functionality

Eric Dumazet
2007-04-03T21:10:14Z

Rik van Riel a A(C)crit :
> Andrew Morton wrote:
>
>> Oh. I was assuming that we'd want to unmap these pages from
>> pagetables and
>> mark then super-easily-reclaimable. So a later touch would incur a minor
>> fault.
>>
>> But you think that we should leave them mapped into pagetables so no such
>> fault occurs.
>
>> Leaving the pages mapped into pagetables means that they are considerably
>> less likely to be reclaimed.
>
> If we move the pages to a place where they are very likely to be
> reclaimed quickly (end of the inactive list, or a separate
> reclaim list) and clear the dirty and referenced lists, we can
> both reclaim the page easily *and* avoid the page fault penalty.
>

There is one possible speedup :

- If an user app does a madvise(MADV_DONTNEED), we can assume the pages can
later be bring back without need to zero them. The application doesnt care.

A page fault is not that expensive. But clearing N*PAGE_SIZE bytes is, because
it potentially evicts a large part of CPU cache.

If I recall well, mysql bench Ulrich mentioned was allocating/freeing large
areas (100 Kbytes or so) in a loop.

mmap()/brk() must give fresh NULL pages, but maybe madvise(MADV_DONTNEED) can
relax this requirement (if the pages were reclaimed, then a page fault could
bring a new page with random content)