lkml.org 
[lkml]   [2006]   [Jan]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] Shrinks sizeof(files_struct) and better layout
Andi Kleen a écrit :
>
> Total data of all objects together. That's because caches always get their
> own pages and cannot share them with other caches.

OK for this part.

> The overhead of the kmem_cache_t by itself is negligible.

This seems a common misconception among kernel devs (even the best ones Andi :) )

On SMP (and/or NUMA) machines : overhead of kmem_cache_t is *big*

See enable_cpucache in mm/slab.c for 'limit' determination :

if (cachep->objsize > 131072)
limit = 1;
else if (cachep->objsize > PAGE_SIZE)
limit = 8;
else if (cachep->objsize > 1024)
limit = 24;
else if (cachep->objsize > 256)
limit = 54;
else
limit = 120;

On a 64 bits machines, 120*sizeof(void*) = 120*8 = 960

So for small objects (<= 256 bytes), you end with a sizeof(array_cache) = 1024
bytes per cpu

If 16 CPUS : 16*1024 = 16 Kbytes + all other kmem_cache structures : (If you
have a lot of Memory Nodes, then it can be *very* big too).

If you know that no more than 100 objects are used in 99% of setups, then a
dedicated cache is overkill, even locking 100 pages because of extreme
fragmentation is better.

Probability that a *lot* of tasks are created at once and killed at once is
close to 0 during a machine lifetime.


Maybe we can introduce an ultra basic memory allocator for such objects
(without CPU caches, node caches), so that the memory overhead is small.
Hitting a spinlock at thread creation/deletion time is not that time critical.

Eric
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2006-01-04 12:44    [W:0.058 / U:0.508 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site