[patch] 2.4's sys_readahead is borked

[patch] 2.4's sys_readahead is borked

[Robert Love]

In 2.4, sys_readahead() performs readahead against a maximum of half the
number of inactive pages.  This is dumb, as it ignores the number of
free pages completely.  Worse, in certain situations, such as boot, the
inactive list can be quite small and the free list quite large, but
readahead(2) won't do anything.

The right thing to do is limit sys_readahead() to a maximum of half of
the sum of the number of free pages and inactive pages, which is what
2.6 does.

Attached patch is against 2.4.25-pre8.  Please apply.

	Robert Love

 mm/filemap.c |    2 +-
 1 files changed, 1 insertion(+), 1 deletion(-)

diff -urN linux-2.4.25-pre8/mm/filemap.c.orig linux-2.4.25-pre8/mm/filemap.c
--- linux-2.4.25-pre8/mm/filemap.c.orig	2004-02-03 19:13:33.540115456 -0500
+++ linux-2.4.25-pre8/mm/filemap.c	2004-02-03 19:13:49.468693944 -0500
@@ -1965,7 +1965,7 @@
 		nr = max;
 
 	/* And limit it to a sane percentage of the inactive list.. */
-	max = nr_inactive_pages / 2;
+	max = (nr_free_pages() + nr_inactive_pages) / 2;
 	if (nr > max)
 		nr = max;
 

Re: [patch] 2.4's sys_readahead is borked

[Marcelo Tosatti]

On Tue, 3 Feb 2004, Robert Love wrote:

> In 2.4, sys_readahead() performs readahead against a maximum of half the
> number of inactive pages.  This is dumb, as it ignores the number of
> free pages completely.  Worse, in certain situations, such as boot, the
> inactive list can be quite small and the free list quite large, but
> readahead(2) won't do anything.
>
> The right thing to do is limit sys_readahead() to a maximum of half of
> the sum of the number of free pages and inactive pages, which is what
> 2.6 does.

Hi Robert,

This looks okay, applied.

Question: Do you know any user of sys_readahead() ?

Re: [patch] 2.4's sys_readahead is borked

[Robert Love]

On Wed, 2004-02-04 at 12:30 -0200, Marcelo Tosatti wrote:

Hi, Marcelo.

> Hi Robert,
> 
> This looks okay, applied.

Thanks!

> Question: Do you know any user of sys_readahead() ?

Not really - I've been playing with it.  But OpenOffice just added it to
preload some of their libraries.  It should probably be deprecated and
remove in 2.7, since posix_fadvise(POSIX_FADV_WILLNEED) does this same
thing.

	Robert Love

Re: [patch] 2.4's sys_readahead is borked

[Mike Fedyk]

Robert Love wrote:
> On Wed, 2004-02-04 at 12:30 -0200, Marcelo Tosatti wrote:
>>Question: Do you know any user of sys_readahead() ?
> 
> 
> Not really - I've been playing with it.  But OpenOffice just added it to
> preload some of their libraries.  It should probably be deprecated and
> remove in 2.7, since posix_fadvise(POSIX_FADV_WILLNEED) does this same
> thing.

In 2.4 also?

Re: [patch] 2.4's sys_readahead is borked

[Robert Love]

On Wed, 2004-02-04 at 10:42 -0800, Mike Fedyk wrote:

> Robert Love wrote:
>
> > Not really - I've been playing with it.  But OpenOffice just added it to
> > preload some of their libraries.  It should probably be deprecated and
> > remove in 2.7, since posix_fadvise(POSIX_FADV_WILLNEED) does this same
> > thing.
> 
> In 2.4 also?

No, posix_fadvise() is only in 2.6.

So we deprecate readahead() in 2.7...

	Robert Love

Re: [patch] 2.4's sys_readahead is borked

[Arjan van de Ven]

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On Wed, 2004-02-04 at 09:30, Marcelo Tosatti wrote:

> 
> Question: Do you know any user of sys_readahead() ?

Development versions of Fedora will sys_readahead a bunch of stuff
during system boot to avoid disk IO (and seeks) later on. The plan is to
expand this further from the current code (eg readahead stuff while dhcp
is trying to get an address and the disk is idle, etc etc).



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