Re: Meaning of "Page faults with physical IO" ?

The comments so far have been about what paging/swapping figures mean in
a "traditional" context, but it's worth bearing in mind that on some systems
(Sun's Solaris 2, at least), the virtual memory and filesystem handling are
unified and what a user process sees as reading or writing a file, the system
simply sees as paging something in from disk or a page being updated so it
needs to be paged out. [I suppose you could view it as similar to the operating
system memory-mapping the files behind-the-scenes.]

The effect of this is that on Solaris 2, paging figures will also include file
I/O. Or rather, the figures from vmstat certainly appear to include file I/O,
and I presume (but can't quickly test) that figures such as those quoted by
Squid will also include file I/O.

To confirm the above (which represents an impression from what I've read and
observed, rather than 100% certain facts...), using an otherwise idle Sun Ultra
1 system system I just tried using cat (small, shouldn't need to page) to copy
(a) one file to another, (b) a file to /dev/null, (c) /dev/zero to a file, and
(d) /dev/zero to /dev/null (interrupting the last two with control-C after a
while!), while watching with vmstat. 300-600 page-ins or page-outs per second
when reading or writing a file (rather than a device), essentially zero in
other cases (and when not cat-ing).

So ... beware assuming that all systems are similar and that paging figures
represent *only* program code and data being shuffled to/from disk - they
may also include the work in reading/writing all those files you were
accessing...

                                John Line

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