me at mgoldman.com
Thu Jan 28 13:09:08 CET 2010
My backing store is file-based. In any case, thanks to you both -- I have
ordered some more RAM and will see what happens.
> On Wed, Jan 27, 2010 at 12:23 AM, Michael Fischer <michael at dynamine.net>wrote:
>> On Tue, Jan 26, 2010 at 7:23 PM, Martin Goldman <me at mgoldman.com> 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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