Re: cache misses in gcc 3.3

[Andi Kleen <ak at muc dot de>]

On Sun, Feb 09, 2003 at 11:31:44PM +0100, Daniel Berlin wrote:
> 
> 
> On Sun, 9 Feb 2003, Andi Kleen wrote:
> 
> >
> > I ran a slightly older gcc 3.3 on linux in cachegrind
> > (http://developer.kde.org/~sewardj/) on an Athlon with 256K cache.
> > cachegrind is a cache simulator and gives detailed reports about
> > the caching behaviour of a program.
> 
> Cachegrind usually gives me *wildly* different results than real profiling
> (either with oprofile or vtune for linux).

I double checked with oprofile now (ordering the other way round)

For dcaches it is not that much different - most of the functions
idenitified as hot by cachegrind are hot in oprofile too.

I guess the difference can be explained in the difference methologies.
cachegrind runs an exact model, while oprofile does an statistical
sample (i used an NMI every 2000 events) and has to fight with an 
out-of-order CPU with a large issue window mixing instructions too.

New interesting is lookup_tag(). I'm not sure why it gets 
that many dcaches misses and isn't noticed by cachegrind at all.
Could be an OOO artifact.

yyparse/bison slowness is even more clearly visible here.

-Andi

Cpu type: Athlon
Cpu speed was (MHz estimation) : 896.198
Counter 0 counted DATA_CACHE_MISSES events (Data cache misses) with a unit mask of 0x00 (Not set) count 2000
vma      samples  %-age       symbol name             linenr info
[...]
0808b6e0 64       1.48561     init_alias_analysis     /src/src/gcc-3.3/obj/gcc/../../gcc/gcc/alias.c:2692
081fae60 65       1.50882     side_effects_p          /src/src/gcc-3.3/obj/gcc/../../gcc/gcc/rtlanal.c:2256
0822ca50 76       1.76416     ggc_alloc               /src/src/gcc-3.3/obj/gcc/../../gcc/gcc/ggc-page.c:872
08112560 85       1.97307     gt_ggc_mx_rtx_def       /src/src/gcc-3.3/obj/gcc/gtype-desc.c:374
081fa1b0 86       1.99629     note_stores             /src/src/gcc-3.3/obj/gcc/../../gcc/gcc/rtlanal.c:1610
08049610 143      3.31941     yyparse                 /usr/share/bison/bison.simple:383
0822d100 188      4.36397     ggc_pop_context         /src/src/gcc-3.3/obj/gcc/../../gcc/gcc/ggc-page.c:1257
0822cc10 304      7.05664     ggc_set_mark            /src/src/gcc-3.3/obj/gcc/../../gcc/gcc/ggc-page.c:1002
0805a2d0 327      7.59053     gt_ggc_mx_lang_tree_node /src/src/gcc-3.3/obj/gcc/gt-c-decl.h:44
080543e0 363      8.42618     lookup_tag              /src/src/gcc-3.3/obj/gcc/../../gcc/gcc/c-decl.c:2343