Re: Reducing inode cache usage on 2.4?

[Andrea Arcangeli <andrea@suse.de>]

On Mon, Dec 20, 2004 at 01:06:34PM -0200, Marcelo Tosatti wrote:
> The thing is right now we dont try to reclaim from icache/dcache _at all_ 
> if enough clean pagecache pages are found and reclaimed.
> 
> Its sounds unfair to me.

If most ram is in pagecache there's not much point to shrink the dcache.
The more ram goes into dcache/icache, the less ram will be in pagecache,
and the more likely we'll start shrinking dcache/icache. Also keep in
mind in a highmem machine the pagecache will be in highmemory and the
dcache/icache in lowmemory (on very very big boxes the lowmem_reserve
algorithm pratically splits the two in non-overkapping zones), so
especially on a big highmem machine shrinking dcache/icache during a
pagecache allocation (because this is what the workload is doing: only
pagecache allocations) is a worthless effort.

This is the best solution we have right now, but there have been several
discussions in the past on how to shrink dcache/icache. But if we want
to talk on how to change this, we should talk about 2.6/2.7 only IMHO.

> Why not? If we have a lot of them they will probably be hurting performace, which seems
> to be the case now.

The slowdown could be because the icache/dcache hash size is too small.
It signals collisions in the dcache/icache hashtable. 2.6 with bootmem
allocated hashes should be better. Optimizing 2.4 for performance if not
worth the risk IMHO. I would suggest to check if you can reproduce in
2.6, and fix it there, if it's still there.

> Following this logic any workload which generates pagecache and happen
> to, most times, have enough pagecache clean to be reclaimed should not
> reclaim the i/dcache's.  Which is not right.

This mostly happens for cache-polluting-workloads like in this testcase.
If the cache would be activated, there would be less pages in the
inactive list and you had a better chance to invoke the dcache/icache
shrinking.