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SubjectRe: 2.4.8/2.4.9 VM problems


On Tue, 21 Aug 2001, Daniel Phillips wrote:

> On August 20, 2001 10:16 pm, Marcelo Tosatti wrote:
> > On Mon, 20 Aug 2001, Marcelo Tosatti wrote:
> > > On Mon, 20 Aug 2001, Daniel Phillips wrote:
> > > > On Mon, 20 Aug 2001, Marcelo Tosatti wrote:
> > > > > Find riel's message with topic "VM tuning" to linux-mm, then take a look
> > > > > at the 4th aging option.
> > > > >
> > > > > That one _should_ be able to make us remove all kinds of "hacks" to do
> > > > > drop behind, and also it should keep hot/warm active memory _in cache_
> > > > > for more time.
> > > >
> > > > I looked at it yesterday. The problem is, it loses the information about *how*
> > > > a page is used: pagecache lookup via readahead has different implications than
> > > > actual usage.
> >
> > And ah, I forgot something here.
> >
> > Your statement which says "pagecache lookup via readahead has different
> > implications than actual usage" is not really correct.
> >
> > If you only consider "hot" pages as "pages which have been touched",
> > you're going to (potentially) fuck heavy streaming IO workloads.
>
> "Hot" pages are pages that have been touched more than once.
>
> The idea of use-once (on the read side) is to retain the readahead
> pages just long enough to use them, and not a lot longer.
>
> If you've seen streaming IO pages getting evicted before being used,
> I'd like to know about it because something is broken in that case.

I've seen the first page read by "swapin_readahead()" (which is the actual
page we want to swapin) be evicted _before_ we could actually use it (so
the read_swap_cache_async() call had to read the same page _again_ from
disk).


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