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To: "M. Edward (Ed) Borasky" <znmeb@aracnet.com>
Subject: Re: [2.4.17/18pre] VM and swap - it's really unusable
Date: Sun, 6 Jan 2002 13:51:58 -0200
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On 5 January 2002 10:59, M. Edward (Ed) Borasky wrote:

> OK, here's some pseduo-code for a real-world test case. I haven't had a
> chance to code it up, but I'm guessing I know what it's going to do. I'd
> *love* to be proved wrong :).
>
> # build and boot a kernel with "Magic SysRq" turned on
> # echo  1 > /proc/sys/kernel/sysrq
> # fire up "nice --19 top" as "root"
> # read "MemTotal" from /proc/meminfo
>
> # now start the next two jobs concurrently
>
> # write a disk file with "MemTotal" data or more in it

Like dd if=/dev/zero of=/tmp/file bs=... count=... ?

> # perform a 2D in-place FFT of total size at least "MemTotal/2" but less
> # than "MemTotal"

I'm willing to try. What program can I use for FFT?

> What's worse is if the page cache gets so big that the FFT has to start
> swapping. For those who aren't familiar with 2D FFTs, they take two
> passes over the data. The first pass will be unit strides -- sequential
> addresses. But the second pass will be large strides -- a power of two.
> That second pass is going to be brutal if every page it hits has to be
> swapped in!

Can you describe FFT memory access pattern in more detail?
I'd like to write a simple testcase with similar 'bad' pattern.
--
vda
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