linux/arch/powerpc/oprofile
Nick Piggin c2452f3278 shrink struct dentry
struct dentry is one of the most critical structures in the kernel. So it's
sad to see it going neglected.

With CONFIG_PROFILING turned on (which is probably the common case at least
for distros and kernel developers), sizeof(struct dcache) == 208 here
(64-bit). This gives 19 objects per slab.

I packed d_mounted into a hole, and took another 4 bytes off the inline
name length to take the padding out from the end of the structure. This
shinks it to 200 bytes. I could have gone the other way and increased the
length to 40, but I'm aiming for a magic number, read on...

I then got rid of the d_cookie pointer. This shrinks it to 192 bytes. Rant:
why was this ever a good idea? The cookie system should increase its hash
size or use a tree or something if lookups are a problem. Also the "fast
dcookie lookups" in oprofile should be moved into the dcookie code -- how
can oprofile possibly care about the dcookie_mutex? It gets dropped after
get_dcookie() returns so it can't be providing any sort of protection.

At 192 bytes, 21 objects fit into a 4K page, saving about 3MB on my system
with ~140 000 entries allocated. 192 is also a multiple of 64, so we get
nice cacheline alignment on 64 and 32 byte line systems -- any given dentry
will now require 3 cachelines to touch all fields wheras previously it
would require 4.

I know the inline name size was chosen quite carefully, however with the
reduction in cacheline footprint, it should actually be just about as fast
to do a name lookup for a 36 character name as it was before the patch (and
faster for other sizes). The memory footprint savings for names which are
<= 32 or > 36 bytes long should more than make up for the memory cost for
33-36 byte names.

Performance is a feature...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
2008-12-31 18:07:38 -05:00
..
cell shrink struct dentry 2008-12-31 18:07:38 -05:00
backtrace.c
common.c on_each_cpu(): kill unused 'retry' parameter 2008-06-26 11:24:38 +02:00
Makefile [POWERPC] Made FSL Book-E PMC support more generic 2008-02-05 23:34:14 -06:00
op_model_7450.c [CELL] oprofile: add support to OProfile for profiling CELL BE SPUs 2007-07-20 21:42:24 +02:00
op_model_cell.c powerpc/cell/OProfile: Fix on-stack array size in activate spu profiling function 2008-10-31 16:13:51 +11:00
op_model_fsl_emb.c [POWERPC] Made FSL Book-E PMC support more generic 2008-02-05 23:34:14 -06:00
op_model_pa6t.c [CELL] oprofile: add support to OProfile for profiling CELL BE SPUs 2007-07-20 21:42:24 +02:00
op_model_power4.c powerpc: Expose PMCs & cache topology in sysfs on 32-bit 2008-08-20 16:34:58 +10:00
op_model_rs64.c [CELL] oprofile: add support to OProfile for profiling CELL BE SPUs 2007-07-20 21:42:24 +02:00