Date: Thu, 15 Feb 1996 14:02:41 +0100 (MET) From: Luigi Rizzo <luigi@labinfo.iet.unipi.it> To: hackers@freebsd.org Subject: Paging questions Message-ID: <199602151302.OAA07491@labinfo.iet.unipi.it>
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More questions/thoughts on paging support for small-memory system.
First, a preliminary question:
How expensive is it to get a page fault which immediately
returns without doing anything (lets call this a Quick Fault)
? According to the 4.3BSD book, I believe this is what was used
on the VAX to simulate the 'reference' bit for a page. The x86
supports this in hardware (the DIRTY bit) so we do not have
such a need. However I would not expect this kind of operation
to be very expensive.
And now a couple of questions/ideas:
1)
Assume a page is swapped to disk, then copied back in core,
and not modified afterwards. Do we keep a reference to the disk
block for the page, so as to save subsequent pageouts ? Do we
have an idea on how frequently this occurs ? I have no data on this.
2)
Can we make DATA pages shared, CopyOnWrite more or less as text
pages ? The idea is to consider DATA page readonly when they are
first loaded, and make them shared. At the first write access, the
page is either copied or (if there is only one process referencing
it) transformed into a RW page -- this should be the way COW works.
This could be useful in the following cases:
- "undump"-ed programs;
- "crunched" executables
- shared libraries
I have to admit I don't know if this can be really useful. A quick
check on my system shows that many standard binaries and libraries
only use 1-2 pages of DATA and so this is not going to save much
memory. However, it should not introduce a significant overhead in
the system (an additional Quick Fault in all those cases a DATA
page would be copied from disk).
Thanks
Luigi
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Luigi Rizzo Dip. di Ingegneria dell'Informazione
email: luigi@iet.unipi.it Universita' di Pisa
tel: +39-50-568533 via Diotisalvi 2, 56126 PISA (Italy)
fax: +39-50-568522 http://www.iet.unipi.it/~luigi/
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