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SubjectRe: More cluuse, was: Re: Wierd IDE/Triton behavior
Date
On Fri, 10 Jan 1997 19:31:56 GMT, you wrote:

> On Fri, 10 Jan 1997 15:36:09 +0100, you wrote:
>
> > Hallo,
> >
> > I have investigated more on the effect of decreased speed with EIDE drives
> > at least with Triton I and II boards on the recent kernel.
> >
> > [skipped]
>
> I can confirm that something happened after 2.0.22.
>
> I said "dd if=/dev/hda of=/dev/null bs=1024k count=100" on a drive with
> no mounted filesystems using a 2.0.22, a 2.0.27, and a 2.1.20 kernel. I
> also said "hdparm -tT /dev/hda" as a check.
>
> Here are the results:
>
> kernel | time for 100MB | MB/s | MB/s (hdparm)
> ---------+----------------+------+------------------
> 2.0.22 | 88s | 1.14 | 1.14
> 2.0.27 | 166s | 0.60 | 0.65
> 2.1.20 | 87 | 1.15 | 1.16
> | | |
>
> [skipped]
>

Further testing reveals that this is probably not a disk problem but
a memory management one.

Immediately after booting the 2.0.27 kernel, I can reproduce the slow
results every time. If I run the system for a while doing something
like compiling a kernel or running X or generally churning free memory,
the problem disappears! From then on, the test runs as fast as with the
other kernels. Interesting.

The kernel size also seems important. I built a special kernel (with
profiling support) to test this phenomenon, but it does not exhibit the
behavior. Other 2.0.27 kernels I have (with different configurations)
don't show this slowdown.

Luckily, I found out that you can profile a kernel even it if wasn't
originally compiled with profiling support. (A definite feature.) One
very obvious difference between a slow run and a fast one is that
shrink_specific_buffers is called 6970 times in a slow run but *never*
in a fast one.

I know nothing about Linux memory management, but it appears that there
are certain initial conditions that can cause big performance hits.

Any experts have any ideas?


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