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SubjectRe: 2.5.2-pre performance degradation on an old 486 (it's the scheduler)
On Sat, 5 Jan 2002, Mikael Pettersson wrote:

> On Sat, 5 Jan 2002 09:25:48 +0100 (CET), Matthias Hanisch wrote:
> >On Sat, 5 Jan 2002, Mikael Pettersson wrote:
> >
> >> When running 2.5.2-pre7 on my old for-testing-only 486(*),
> >> file-system accesses seem to come in distinct bursts preceded
> >> by lengthy pauses. Overall performance is down quite significantly
> >> compared to 2.4.18pre1 and 2.2.20pre2. To measure it I ran two
> >> simple tests:
> >>
> >> Test 1: time to boot the kernel, from hitting enter at the LILO
> >> prompt to getting a login prompt
> >> Test 2: time to "rm -rf" a clean linux-2.4.17 source tree, using
> >> the newly booted kernel (no other access to the tree before that,
> >> so it wasn't cached in any way, and the machine was otherwise idle)
> >>
> >> Test 1 Test 2
> >> 2.2.21pre2: 71 sec 75 sec
> >> 2.4.18pre1: 64 sec 72 sec
> >> 2.5.2-pre7: 97 sec 251 sec
> >>
> >> I haven't noticed any slowdowns on my other boxes, so I didn't
> >> do any measurements on them. On the 486 it's very very obvious.
> >
> >This is exactly, what I see with my old 486 box. It started with
> >2.5.2-pre3, which contained two major items:
> >
> >- bio changes from Jens
> >- scheduler changes from Davide
> >
> >Surprisingly, backing out the bio changes didn't help. Backing out the
> >scheduler changes from Davide did!!
>
> BINGO! Running 2.5.2-pre8 with the scheduler changes backed out made
> all the difference! Interactive responsiveness is much improved and
> performance in the above two tests I ran is back to 2.4.18pre1 levels.
>
> With 2.5.2-pre8 vanilla the 486 is getting large variation in Test 2
> above (157s, 237s, 292s), but is never even close to 2.2/2.4 levels.
>
> >> (*) 100MHz 486DX4, 28MB ram, no L2 cache, two old and slow IDE disks,
> >> small custom no-nonsense RedHat 7.2, kernels compiled with gcc 2.95.3.

Are you able to profile the kernel ?




- Davide


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