Re: [PATCH] mem notifications v2

[Rik van Riel <riel@redhat.com>]

On Fri, 23 Nov 2007 01:27:38 +0100
"Daniel SpAJPYng" <daniel.spang@gmail.com> wrote:

> On 11/22/07, Rik van Riel <riel@redhat.com> wrote:
> > On Thu, 22 Nov 2007 12:23:55 +0100
> > "Daniel SpAJPYng" <daniel.spang@gmail.com> wrote:
> >
> > > When the page cache is filled, the notification is a bit early as the
> > > following example shows on a small system with 64 MB ram and no swap.
> > > On the first run the application can use 58 MB of anonymous pages
> > > before notification is sent. Then after the page cache is filled the
> > > test application is runned again and is only able to use 49 MB before
> > > being notified.
> >
> > Excellent.  Throwing away useless memory when three is still
> > useful memory available sounds like a good idea.
> >
> > > I see it as a feature to be able to throw out inactive binaries and
> > > mmaped files before getting notified about low memory.
> >
> > I think that once you get low on memory, you want a bit of
> > both.  Inactive binaries and mmaped files are potentially
> > useful; in-process free()d memory and caches are just as
> > potentially (dubiously) useful.
> >
> > Freeing a bit of both will probably provide a good compromise
> > between CPU and memory efficiency.
> 
> I get your point, but strictly speaking, it is never freeing inactive
> binaries nor mapped files until all in-process cache are freed. But
> your argument is still valid, although a tad weaker, if you replace
> ``inactive binaries and mmaped files'' with ``page cache''.

How can you say that when you do not know how many userland
processes will get woken up, or how much memory they will
free?

The kernel sends the notification in *addition* to freeing
page cache, not instead of freeing page cache.

-- 
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it." - Brian W. Kernighan

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