Re: Memory question
Martin Gregorie wrote On 05/03/07 17:05,:
It's certainly possible that the JVM explicitly
releases virtual memory under some conditions, but few
applications find it worthwhile to do so.
As I said, I wonder if it would start doing that once its working page
set starts being swapped out to disk. It could help to improve
performance if the CG was to return garbage-collected pages. Reducing
the size of virtual memory offers a chance of fitting the entire program
inside physical RAM again. This should tend to reduce page swapping.
(See also Mark Thornton's response, describing how to
get HotSpot to release memory to the O/S.)
Releasing virtual memory doesn't by itself do a lot to
reduce paging pressure. If the system as a whole is short
on memory, the stuff that goes out to disk will (should) be
the stuff the programs aren't actively using. If Process P
owns 500MB but is only actively using 100MB, the other 400MB
should migrate to the swap device and sit there idly; the
only effect of releasing the VM would be to make more swap
space available, not to decrease the paging rates.
Of course, P might well change its behavior if it knew
it were running with just 100MB instead of 500MB -- for
example, it would touch the other 400MB "not at all" instead
of "seldom." But even that effect could be achieved without
actually giving up the 400MB: just stop touching it for a
while, and it will migrate out to swap. It's a lot easier
to stop touching the extra space if you actually let go of
it (because the O/S will slap your wrist if you get careless),
but it's the change in memory access patterns, not the fact
of jettisoning the VM, that affects paging behavior.
Also, it's difficult to make system-wide policy decisions
from inside the confines of a single process. P might well
observe that paging rates are rising, but what should P's
response be? P is ignorant of what else is running on the
box; paging rates might be going up because Process Q has
become greedy. Should P flush its caches and go on a memory
diet for Q's sake? That would seem to depend on the relative
importance of P and Q in the esteem of the system operator,
something neither P nor Q has much knowledge of. It seems to
me that most decisions P might try to make in this regard
would be uninformed guesswork.
The option Mark Thornton mentions is, in its way, a means
of telling P something about its relative importance in the
grand scheme of things, and helping it with its decision of
how to consume resources. (Although it doesn't seem that
HotSpot actually monitors paging rates to make the decision;
it instead acts when it perceives an embarasse de richesse.)
--
Eric.Sosman@sun.com