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SubjectRe: swap cache
I think Rama and I are talking about something different from what
Rik is talking about. Rather than using compression (only) to
increase effective disk bandwidth, a fraction of RAM can be used
to hold compressed pages, and avoid paging those out to disk and
paging them back in again.

If it is true that the dominant cost of paging is in seeks (and
that seems likely) then a compressed page cache is a good idea;
it's easy to compress and uncompress a page many times faster
than a seek.

BTW, does anybody have suggestions for how to do precise timings
inside or outside of the kernel? One of my students wrote some
routines that read the Pentium-family instruction-cycle counter,
so that we can time things down to a few nanoseconds. (This
is how we measure the speed of our compression algorithms in
userland.) If there are patches to do this in a better way, though,
I'd be very interested. (Or if somebody wanted to use my student's
code to implement hi-res time as a patch---maybe implementing
Solaris-like gethrtime---Mark gives it away free.)

Are there any plans to support hi-res time in the kernel? I think
all the new processors have cycle timers, and that sort of thing
is very handy for measuring the performance of system code.


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