Re: OT: question about sizing swap for solaris
- From: Mladen Gogala <mladen@xxxxxxxxxxxxxxx>
- To: oracle-l@xxxxxxxxxxxxx
- Date: Tue, 4 May 2004 09:33:00 -0400
On 05/04/2004 02:18:32 AM, zhu chao wrote:
> Hi, Cary:
> The explanation sounds reasonable. But does solaris use this FIFO to
> manage its VM? If not , more swap space should not cause hard pageout to
> occur more easily, right? Is there any unix that used this kind of VM
> management ?
Belady anomaly is a nice spice to a computer science course but, in practice,
I know of no such OS. All operating systems that I work with, with possible
exception of one (win 2k), use some kind of LRU algorithm. The rationale behind
the LRU algorithm is that if the page hasn't been used for quite a while, it is
quite unlikely to be used again, in the near future. That is why students of
computer engineering are thought of another important technique to make
things right: locality of references. The "locality of references principle"
states that the execution points that are close to each other in time should
also operate on the set of pages that are close to each other in memory. Good
way
of breaking away from the "locality of references" principle is to use object
orientation, where you don't really know where are the methods stored or what
routines do they actually call. What is more, because of the name mangling
(talking C++ now), things like prof, gprof or pixie are useless with OO stuff.
The efficienc of the LRU algorithm is based on the assumption that the vast
majority of programs will respect the "locality of reference". By the advent
of OO, many operating systems (Linux is an example) are shifting to MFU (most
frequently used) algorithms. That is one of the differences between
Arcangelli's VM
engine and Rick van Rein's one. The other difference is that the Arcangelli's
engine
is much better suited for the NUME hardware.
Oracle did so with their "touch count" policy which,basically, keeps in memory
the
buffers with the highest touch count, i.e., "the most frequently used" ones.
Also,
the "two handed clock" algorithm for the page replacement is being modified to
become
more "MFU" oriented, especially in Slowaris. Now "bflushd" passes do not turn
off
"referenced" bits, they're counting "touches". So, the "OO" approach comes with
the
"FU" virtual memory engines.
--
Mladen Gogala
Oracle DBA
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