Messages in this thread | | | Date | Sun, 19 Oct 2003 18:05:00 +0200 | From | "Ihar 'Philips' Filipau" <> | Subject | Re: [2.6 patch] add a config option for -Os compilation |
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Martin J. Bligh wrote: > > But if someone with a small cache would actually *measure* the damned > thing, I'd be more impressed ... I've never seen that, but perhaps > I just missed it. > > Point is the same either way though ... we shouldn't unconditionally > optimise for *anyone's* system. If it's faster on all systems that anyone > can be bothered to measure, great. If it's faster on some, and slower on > others, a config option seems more appropriate, defaulting to the majority > of users. >
The thing is, that in fact kernel optimization is not that important.
The goal of kernel is to provide framework for applications to the job well. I wasn't doing any kernels measurements - since kernel docs are saying that -O2 is the standard. And it is really hard to measure kernel only perfomance. (And it is rather pointless - I'm not going to sell linux kernel ;-)))
But indeed I was testing my application on embedded system with 16K L1 cache. Results were pretty predictable: gcc 2.95.3 + -Os was giving some (around 2-3%) performance improvements, while 2.95.3 + -O[23] and 3.2.3 with any optimization were giving aprox. the same times. (App is bloated with third-party libraries, C++ and threads. Save God I have killed all exceptions - they were really really really slow on target system.)
But on other side - since embedded systems are not that overclocked as high-end toys - cache miss is not that painful. As of docs, NatSemi Geode@266MHz cache miss costs exactly 266/66 == 4 cycles.
-- Ihar 'Philips' Filipau / with best regards from Saarbruecken. -- "... and for $64000 question, could you get yourself vaguely familiar with the notion of on-topic posting?" -- Al Viro @ LKML
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