Re: 2.6.6-rc3-mm2 (4KSTACK)

[Andrew Morton]

Horst von Brand <vonbrand@xxxxxxxxxxxx> wrote:
>
> Bill Davidsen <davidsen@xxxxxxx> said:
> 
> [...]
> 
> > I tried 4k stack, I couldn't measure any improvement in anything (as in 
> > no visible speedup or saving in memory).
> 
> 4K stacks lets the kernel create new threads/processes as long as there is
> free memory; with 8K stacks it needs two consecutive free page frames in
> physical memory, when memory is fragmented (and large) they are hard to
> come by...

This is true to a surprising extent.  A couple of weeks ago I observed my
256MB box freeing over 20MB of pages before it could successfully acquire a
single 1-order page.

That was during an updatedb run.

And a 1-order GFP_NOFS allocation was actually livelocking, because
!__GFP_FS allocations aren't allowed to enter dentry reclaim.  Which is why
VFS caches are now forced to use 0-order allocations.


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