Re: [patch] Latency Tracer, voluntary-preempt-2.6.8-rc4-O6

[Paulo Marques <pmarques@grupopie.com>]

Andi Kleen wrote:
>>The final algorithm pre-calculates markers on the compressed symbols so 
>>that the search time is almost divided by the number of markers.
> 
> 
> You could do that at compile time (in scripts/kallsyms.c) 

You're right! I'll see if I can come up with a patch.

If this is done in scripts/kallsyms.c, there is less code needed in the 
actual kernel, and there is no time penalty on the first lookup. This is 
a true win-win scenario :)

Thanks a lot for your suggestion.

>>There are still a few issues with this approach. The biggest issue is 
>>that this is clearly a speed/space trade-off, and maybe we don't want to 
>>waste the space on a code path that is not supposed to be "hot". If this 
>>is the case, I can make a smaller patch, that fixes just the name 
>>"decompression" strcpy's.
> 
> 
> I'm surprised that using 8 markers helps anything. There should 
> be many many more 0 stems than that in a not so big kernel.
> Did you actually measure the hit rate of the cache? I bet it is pretty low.

Well, I only left in the resulting source code the pieces that I 
actually measured to be a significant improvment.

The thing is, this is not exactly a cache.

The problem that the markers address is that the decompression must be 
done sequentially.

If I have 20000 symbols and already figured out from the binary search 
that I need symbol 11201 (for instance), I would have to go through 
11201 symbols to get to the symbol I need.

The algorithm I wrote keeps markers in the middle of the stream, so that 
when I try to fetch symbol 11201, I already know where symbol 10000 is 
because of the marker and only have to go through 1201 symbols.

With N markers you get N times the speed on the name search. (actually 
this is not exactly right for high values of N, because you can only use 
a marker that is a 0 stem, but for N<~32 this is a good approximation)

-- 
Paulo Marques - www.grupopie.com
"In a world without walls and fences who needs windows and gates?"