Re: Running 2.1.129 at extrem load [patch] (Was: Linux-2.1.129..)

Dr. Werner Fink
1998-11-23T22:37:18Z

> > struct page *next_hash;
> > atomic_t count;
> > - unsigned int unused;
> > + unsigned int lifetime;
> > unsigned long flags; /* atomic flags, some possibly updated asynchronously */
>
> Hmm, this looks suspiciously like a new incarnation of
> page aging (which we want to avoid, at least in some
> parts of the kernel).

Yep ... we should avoid removing this `unused'.

> > --- linux-2.1.129/mm/filemap.c Thu Nov 19 20:44:18 1998
> > +++ linux/mm/filemap.c Mon Nov 23 13:38:47 1998
> > @@ -167,15 +167,14 @@
> > case 1:
> > /* is it a swap-cache or page-cache page? */
> > if (page->inode) {
> > - /* Throw swap-cache pages away more aggressively */
> > - if (PageSwapCache(page)) {
> > - delete_from_swap_cache(page);
> > - return 1;
> > - }
> > if (test_and_clear_bit(PG_referenced, &page->flags))
> > break;
> > if (pgcache_under_min())
> > break;
> > + if (PageSwapCache(page)) {
> > + delete_from_swap_cache(page);
> > + return 1;
> > + }
>
> This piece looks good and will result in us keeping swap cached
> pages when the page cache is low. We might want to include this
> in the current kernel tree, together with the removal of the
> free_after construction.

Hmmm ... don't forget the change in __get_free_pages(). Without this
page I see random SIGBUS at extreme load killing random processes.

[...]

> Sorry Werner, but this is exactly the place where we need to
> remove any from of page aging. We can do some kind of aging
> in the swap cache, page cache and buffer cache, but doing
> aging here is just prohibitively expensive and needs to be
> removed.
>
> IMHO a better construction be to have a page->fresh flag
> which would be set on unmapping from swap_out(). Then
> shrink_mmap() would free pages with page->fresh reset
> and reset page->fresh if it is set. This way we can
> free a page at it's second scan so we avoid freeing
> a page that was just unmapped (and giving each page a
> bit of a chance to undergo cheap aging).

Furthermore highly used pages should go not to often
into the swap cache ... this leads to something like
a score list of often used pages. Such a score value
instead of a flag could be easily decreased by
shrink_mmap() scanning all pages.

Werner