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- Newsgroups: comp.sys.sgi.misc
- Path: sparky!uunet!stanford.edu!nntp.Stanford.EDU!dhinds
- From: dhinds@leland.Stanford.EDU (David Hinds)
- Subject: Re: Resident Set Size Confusion
- Message-ID: <1993Jan6.081854.27331@leland.Stanford.EDU>
- Sender: news@leland.Stanford.EDU (Mr News)
- Organization: DSG, Stanford University, CA 94305, USA
- References: <syscrc.726293940@gsusgi1.gsu.edu> <1993Jan6.072923.9471@odin.corp.sgi.com>
- Date: Wed, 6 Jan 93 08:18:54 GMT
- Lines: 53
-
- In article <1993Jan6.072923.9471@odin.corp.sgi.com> blythe@sgi.com (David Blythe) writes:
- >In article <syscrc.726293940@gsusgi1.gsu.edu> syscrc@pickle.gsu.edu (Randy Carpenter) writes:
- >>We have a 4d/380s with 128MB of physical memory. It's used for
- >>computational chemistry. Although there's plenty of available memory,
- >>long running processes never seem to stay completely resident as evident
- >>from the top(1) listing below. Why does this happen?
- >
- >The short answer is that only the recently referenced memory pages stay
- >resident. The OS always keeps track of what is has not being referenced
- >recently and makes these pages available for other uses. Just because
- >you have lots of empty empty space isn't a good reason to keep around a lot
- >of junk you may never look at again.
-
- Really?? As long as there are free pages, I don't think an in-use page
- would ever get swapped out, no matter how long it had been since it was
- last touched. Or do you just mean that transient memory usage by the OS
- might occasionally fill up the really free space and cause some old stuff
- to get swapped out?
-
- >>Ocassionally, these processes will be killed due to insufficient swap
- >>storage although we think there is plenty of available memory. The
- >>current swap is 48MB and we will be expanding it to 128MB soon.
- >>
- >I believe that if you have a process which has grown larger than the size
- >of free swap space and the OS chooses to swap it out, it must swap it out in
- >its entirety and at that point it has no choice but to kill the process.
- >40Mb is a little small, 120Mb is much better but it depends on the mix
- >you run. /etc/swap -l will give you an idea of how much swap space is
- >actually being used ...
-
- Irix shouldn't swap out whole processes at a time -- swapping is done
- page by page.
-
- >>% top
- >>
- >>IRIX chemistry 4.0.1 11150233 IP7 Load[18.03,18.09,18.09] 23:13:15 500 procs
- >> user pid pgrp %cpu proc pri size rss time command
- >> david 5641 381 56.8 5 105 3762 1656 4075:36 sybyl.40exe
- >> hui 7331 278 54.5 0 100 14596 1611 2570:55 charmm21r3exe
- >> tom 22880 15490 45.5 * 104 8390 2578 1060:34 sp_property
- ...and 15 more jobs with rss ~= 1600
-
- If you add up the rss fields, it looks like there are about 30600 4K
- pages of physical memory in use by these large jobs, or about 120 MB.
- So in fact, if this is a typical load, there is no free memory after
- all, since the OS should take up the remaining space. The size fields
- are much larger than the rss's, so most of the memory allocated by
- these jobs must never be touched (otherwise they would overwhelm the
- 48 MB of swap space). This is typical of Fortran programs with large
- static array declarations.
-
- - David Hinds
- dhinds@allegro.stanford.edu
-