Re: swap space: an idea. Please comment.

Hugo Van den Berg (
Mon, 3 Mar 1997 10:06:52 +0100 ()

On Sun, 2 Mar 1997, Herve R.-P. wrote:


> Here, I'm totally anaware of the overhead of such design, or, further
> more, of the impact on existing sources ... Could someone comment ?

Windoze 95 does this, and you know how that performs :-), NT does it to
som extent in the sense that it can grow it's swap to a preset limit.
There are two problems, first of all a runaway process will not only use
up VM, but also your disk. If the disk is your root FS or /tmp fs that
could cause serious problems. Second is the fragmentation you already
mentioned, a progam using large amounts of memory and writing large files
will write file, swap, file, swap or some such pattern, causing swap and
files to be interleaved instead of contiguous.

One thing that will give you a performance hit is the shrinking of the
swap file. When used swap space allows for shrinking it has to be
reorganized because the unused space will almost never be located at the
end of the swap file. On windoze 95 this causes serious thrashing of the
disk for periods ranging from 1 to 10 seconds. The system is almost
unusable during this period. On a multiuser system this is undesirable.

Hugo Van den Berg -
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