Re: excessive swap-in time

phil-news-nospam@xxxxxxxx wrote:
On Mon, 17 Mar 2008 22:08:30 +0000 (UTC) Michel Talon <talon@xxxxxxxxxxxxxxxx> wrote:

|> If there is paging activity, there is some process waiting for it, unless
|> it is some kind of intelligent speculative activity.
|>
|
| Which is exactly the point of a good VM system.

A good VM system should strive to have a minimum of paging activity since
any such activity represents system slowdowns due to page wait.


Not really. The minimum of paging activity is decided by extrernal
factors: the amount of RAM you have bought for the machine, and the load
you put on the machine. Of course RAM is presently insanely cheap, so
you can say it is basically a free resource. It was not the case years
ago, and it will not necessarily the case in the future. Anyways it is
rational to load a server to the point where RAM is fully used and even
a little swap, as long as this does not cause slowdowns. In this
situation, a good VM system manages the necessary paging, and does so in
the least disruptive way, and above all, without trashing. This means
that a page must never be evicted and called back immediately after.
In this respect you are right, a good VM system minimises the paging
activity, under the external constraints. As soon as trashing occurs
the paging activity explodes, and the performance falls down completely.

Of course, if you can manage the load so that no swapping at all occurs,
then the VM performance becomes very secondary. Note that "no swapping at
all" does not mean only that nothing is written to the swap partition.
Another form of paging activity is eviction of pages from text segment
of the binaries (sitting usually under /usr/bin) or libraries, and their
reloading from disk afterwards. So you must have enough memory so that
this last phenomenon never occurs, if you want to play the game of a
totally swapless machine. What would be interesting is to benchmark the
relative perfomance of such a sitaution with the case of a machine with
much less memory and and a reasonable small paging activity. Personnally
i have rarely seen (subjectively) a big difference when boosting the
memory of my desktop or laptop running recent Linux or FreeBSD (i am
not considering Windows, of course).


--

Michel TALON

.

Relevant Pages

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