Date: Fri, 22 Jan 1999 01:38:57 -0500 (EST) From: "John S. Dyson" <dyson@iquest.net> To: hackers@FreeBSD.ORG, dillon@apollo.backplane.org Subject: Error in vm_fault change Message-ID: <199901220638.BAA00257@y.dyson.net>
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The below code removed has an effect almost the opposite of the
complaint that it seems have remedied by removing it. This
code snippet (which was very carefully thought out) provides
a punishment for a process that is grabbing pages from the cache
queue, and allows for THAT process to block, without loosing that
page that is needed. Other processes will be runnable, if they
have their needed memory pages, and not be disturbed. Those
pages accessed by other runnable processes will not be freed
by the pageout daemon (due to the stats that it accumulates.)
Remember, that the cache+free counts is the traditional free
count. This is a way that a process which does massive activations
of free (cache) pages, that process will be throttled. This will
also allow the pageout daemon to do the mapped page stats correctly.
Note that previous discussions regarding proper buffer cache mgmt
of inactive and cache pages (which I came up with a proper solution)
are not appropriate here, since the buffer cache pages do need to
be freed with "lower" priority. The wakeup of the daemon here,
allows the system to catch up with the reactivations. The wakeup
of the daemon in the buffer cache code is silly because the priority
of the buffer cache items should most all of the time be lower
priority, and the sophisticated priority calculations done by the
pageout daemon become specious there.
This is a *critical* feature, and progressive removal of such features
will eventually hurt the already pretty good performance of the VM code
under load.
Please review the comment, because, again, the effect of the code
removal does NOT IN ANY WAY help the situation that is described (in
fact, it can make it worse.)
#if 0
/*
* Code removed. In a low-memory situation (say, a
* memory-bound program is running), the last thing you
* do is starve reactivations for other processes.
* XXX we need to find a better way.
*/
if (((queue - fs.m->pc) == PQ_CACHE) &&
(cnt.v_free_count + cnt.v_cache_count) < cnt.v_free_min) {
vm_page_activate(fs.m);
unlock_and_deallocate(&fs);
VM_WAIT;
goto RetryFault;
}
#endif
--
John | Never try to teach a pig to sing,
dyson@iquest.net | it makes one look stupid
jdyson@nc.com | and it irritates the pig.
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