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Linus Torvalds: 2.4 Kernels Slower Than 2.2 Series on PIII's?

May 03, 2001, 18:58 (8 Talkback[s])
(Other stories by Linus Torvalds)
Date: Wed, 2 May 2001 17:10:31 -0700 (PDT)
From: Linus Torvalds 

On Wed, 2 May 2001, Duc Vianney wrote:
> Has anyone seen performance degradations between 2.2.19 and 2.4.x

Yes.

The signal handling one is because 2.4.x will save off the full SSE2
state, which means that the signal stack is almost 700 bytes, as
compared to <200 before. This is sadly necessary to be able to take
advantage of the SSE2 instructions - and on special applications the
win can be quite noticeable. This one you won't be able to avoid,
although you shouldn't see it on older hardware that do not have
SSE2 (you see it because you have a PIII).

You don't say how much memory you have, but the file handling ones
might be due to a really unfortunate hash thinko that cause the
dentry hash to be pretty much useless on machines that have 512MB of
RAM (it can show up in other cases, but 512M is the case that makes
the hash really become a non-hash). If so, it should be fixed in
2.4.2.

2.4.4 will give noticeably better numbers for fork and fork+exec.
However, the scheduling optimization that does that actually breaks
at least "bash", and it appears that we will just undo it during the
stable series. Even if the bug is obviously in user land (and a fix
is available), stable kernels shouldn't try to hide the problems.

                        Linus