Re: why swap at all?
From: John Bradford
Date: Wed May 26 2004 - 04:18:10 EST
Quote from Roger Luethi <rl@xxxxxxxxxxx>:
> On Wed, 26 May 2004 02:38:23 -0400, Anthony DiSante wrote:
> > Now I buy another 256MB of ram, so I have 512MB of real memory. Why not
> > just disable my swap completely now? I won't have increased my memory's
> > size at all, but won't I have increased its performance lots?
> > Or, to make it more appealing, say I initially had 512MB ram and now I have
> > 1GB. Wouldn't I much rather not use swap at all anymore, in this case, on
> > my desktop?
> Swap serves another (often underrated) purpose: Graceful degradation.
> If you have a reasonably amount of swap space mounted, you will know
> you are running out of RAM because your system will become noticeably
> slower. If you have no swap whatsoever, your first warning will quite
> possibly be an application OOM killed or losing data due to a failed
> memory allocation.
> Think of the slowness of swap as a _feature_.
There is a very negative side to this approach as well, especially if users
allocate excessive swap space.
A run-away process on a server with too much swap can cause it to grind to
almost a complete halt, and become almost compltely unresponsive to remote
If the total amount of storage is just enough for the tasks the server is
expected to deal with, then a run-away process will likely be terminated
quickly stopping it from causing the machine to grind to a halt.
If, on the other hand, there is excessive storage, it can continue running
for a long time, often consuming a lot of CPU.
When the excess storage is physical RAM, this might not be particularly
disasterous, but if it's swap space, it's much more likely to cause a serious
drop in performance.
For a desktop system, it might not be a big deal, but when it's an ISP's server
in a remote data centre, it can create a lot of unnecessary work.
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