Re: [suse-amd64] Puzzling wild variation of execution time

Andi Kleen wrote:

On Thu, 30 Jun 2005 11:02:34 -0700 Sheo Shanker Prasad wrote:

...

(5) My puzzle:

While a computer is well known to be reproducible, the wild variation by a factor of almost 3 in the execution is very disturbing.

It could be cache effects. Linux does not do enforced L2 cache coloring while allocating user memory so in memory intensive programs some variation can happen depending on how well the cache is used. However an Opteron has a 16 way associative L2 cache so such things normally don't happen there.

One difference between 9.1 and 9.3 is that 9.3 allocates the memory from the start to the end after boot up, while 9.1 tended to allocate it from up to down (highest memory first). It is possible that your BIOS has a broken MTRR or somesuch and that some low page is not cached properly and slowing things down. If it was a high page it could be avoided by booting with mem=...M with a number smaller than your memory. However for low pages that's difficult.

-Andi

Is that difference twixt allocation protocols a 2.4.x kernel vs. 2.6.n kerrnel difference (i.e. generic to ANY AMD64 linux) or something else ? -- William A. Mahaffey III --------------------------------------------------------------------- Remember, ignorance is bliss, but willful ignorance is LIBERALISM !!!!