Memory usage

Michael Fischer michael at
Wed Jan 27 06:23:34 CET 2010

On Tue, Jan 26, 2010 at 7:23 PM, Martin Goldman <me at> wrote:

I'm running Varnish on a box with 4GB RAM. There are hundreds of thousands
> of objects being served, and I'm certain that they don't all fit in that
> relatively meager amount of RAM. I understand that Varnish's model dictates
> that the kernel will be trusted to use virtual memory as necessary if the
> cached objects don't fit in RAM. I have a few questions about this:
> 1. How can you tell whether your Varnish objects fit in RAM?

You can't guarantee that they will unless you set your cache size at or
below the amount of RAM you have installed.

> 2. If I have objects residing in virtual memory, to what extent will my
> performance be adversely affected? If I want my site to be fast, do I
> basically need to go out and buy as much RAM as it will take so that virtual
> memory isn't needed?

Technically, it's "go out and buy as much RAM as it will take to avoid being
swamped by paging".  But yes.

> 3. I noticed tonight that my machine was using a few hundred megs of swap
> space, which I've never seen happen before. Varnish is the only non-system
> service running on this box. My understanding was that Varnish would get
> only as much RAM as was available and then send the overflow into the
> file-backed virtual memory. If that's the case, though, then why is swap
> space being used? Is this just a side effect of how the kernel allocates
> memory, or is something else going on here?

Is your backing store file-based, or malloc-based?  If the latter, that
would explain the swap space being consumed.  Or, as Darryl said, the
housekeeping overhead of a VERY large file-backed cache could make the
Varnish process very large.

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