Thanks Darryl! One follow-up about the VIRT size (mine is 55.2GB). I thought the VIRT size includes the entire amount of VARNISH_STORAGE_SIZE (50GB in my case), regardless of how much of the virtual memory is actually being used to store cached objects. This seems to be the case based on a few minutes of experimenting with that setting. So I'm not sure I understand how to determine the amount of virtual memory I'm actually using -- in other words, the amount of RAM I need to add for optimal performance -- from the VIRT and RES numbers alone. Any chance I could ask you to please fill in what I'm missing?
Thanks again, Martin On Tue, Jan 26, 2010 at 10:34 PM, Darryl Dixon - Winterhouse Consulting < darryl.di...@winterhouseconsulting.com> wrote: > Hi Martin, > > > 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? > > In short, `top` - the VIRT column tells you total virtual process size, > the RES column then tells you which portion of that is currently resident > in physical memory > > > 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? > > Pretty much. > > > 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? > > Two things; > 1) The varnish process itself requires memory (eg, to hold the ban list > etc), which is not part of the file-backed object cache. > 2) Even if the above usage were minimal, it is still entirely possibly > that your VMM (the OS) has decided that the memory being used to cache > objects is more important that some other system processes that have now > been shunted out to swap. Once again, `top` VIRT versus RES will give you > a good clue > > regards, > Darryl Dixon > Winterhouse Consulting Ltd > http://www.winterhouseconsulting.com >
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