Re: Estrange behaviour of pre9-1

[Rik van Riel <riel@conectiva.com.br>]

On Mon, 15 May 2000, Linus Torvalds wrote:

> The fact that Rik's patch performs so badly is interesting in
> itself, and I thus removed it from my tree.

This may be because the different try_to_free_pages end
up waiting for each other (to complete IO?).

If VM was constructed right, we would never have the
situation where a half-dozen apps are all waiting in
try_to_free_pages() simultaneously.

The bug is, IMHO, the fact that shrink_mmap() frees the
wrong pages by skipping over buffer pages. This causes
"innocent" apps to have pagefaults they didn't deserve
==> slowdown.

> _Most_ of the time when "try_to_free_pages()" is called, the
> memory actually exists, and we call try_to_free_pages() mainly
> because we want to make sure that we don't get into a bad
> situation.

True.

> So, how about doing something like:
> 
>  - if memory is low, allocate the page anyway if you can, but increment a
>    "bad user" count in current->user->mmuse;
>  - when entering __alloc_pages(), if "current->user->mmuse > 0", do a
>    "try_to_free_pages()" if there are any zones that need any help
>    (otherwise just clear this field).
> 
> Think of it as "this user can allocate a few pages, but it's on credit.
> They have to be paid back with the appropriate 'try_to_free_pages()'".

I don't think this will work if we keep stealing the wrong pages
from innocent, small processes with lots of clean pages (ie. bash,
vi, emacs, ...).

> Rik? I think this would solve the fairness concerns without the
> need to tell the rest of the world about a process trying to
> free up memory and causing bad performance..

The main problem now seems to be bad page replacement and a
practically unbounded wait time inside try_to_free_pages().

If we fix those, I think it should be possible to move back
to a slightly more conservative (and safe) model (like what
I had in my patch ... you might argue it is too conservative,
slow or whatever, but it should be the more robust one).

regards,

Rik
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