Re: Interesting item came up while working on FreeBSD's pageout daemon

James Antill wrote:

quoted

To estimate the cost of paging io you have to think in terms of the
extra work you have to do because you don't have infinite memory.  In
other words, you would have had to write those dirty pages anyway - this
is an unavoidable cost.  You incur an avoidable cost when you reclaim a
page that will be needed again sooner than some other candidate.  If the
page was clean the cost is an extra read, if dirty it's a write plus a
read.  Alternatively, the dirty page might be written again soon - if
it's a partial page write the cost is an extra read and a write, if it's
a full page the cost is just a write.  So it costs at most twice as much
to guess wrong about a dirty vs clean page.  This difference is
significant, but it's not as big as the 1 usec vs 5 msec you suggesed.

 As I understand it you can't just add the costs of the reads and
writes as 1 each. So given...

 Clean = 1r
 Dirty = 1w + 1r

...it's assumed that a 1w is >= than a 1r, but what are the exact
values ?

By read and write I am talking about the necessary transfers to disk,
not the higher level file IO.  Transfers to and from disk are nearly
equal in cost.

 It probably gets even more complex as if the dirty page is touched
between the write and the cleanup then it'll avoid the re-read
behavior and will appear faster (although it slowed the system down a
little doing it's write).

Oh yes, it gets more complex.  I'm trying to nail down the main costs by
eliminating the constant factors.

quoted

If I'm right then making the dirty page go 3 times around the loop
should result in worse performance vs 2 times.

 It's quite possible, but if there were 2 lists and the dirty pages
were laundered at 33% the rate of the clean pages would that be better
than 50%?

Eh.  I don't know, I was hoping Matt would try it.

--
Daniel
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