Just wanted to follow up with the "solution". It turns out that PURGEs are
being counted against HITs and MISSes now, so that was skewing the data,
and our hit ratio is actually fine, about 83-87%. I was recommended to
calculate the hit ratio using the numbers of requests coming into Varnish
and that it was sending to the backend, i.e. using varnishstat parameters
in collectd, hit-ratio = (client_req - backend_req) / client_req. This
matched up with our pre-1.26 upgrade hit ratio calculations that just used
the cache hits and misses values.
On Mon, Mar 28, 2016 at 9:21 AM, Justin Lloyd <jlloyd.wiki(a)gmail.com> wrote:
Just thought I'd follow up on my original
question, though I've not
resolved it. I did increase the varnish memory from 4 to 8 GB but that
doesn't seem to have had any effect as the hit ratio still tops out around
37%. I did also start tracking Varnish's LRU nuked objects and once it did
start nuking things, it's averaging about 100-200 per minute. Apart from
the MW and extension upgrades, along with dist-upgrading (Ubuntu 12.04) the
servers at the same time to bring everything up-to-date, nothing else about
the content or architecture changed. Any ideas on how to diagnose this
cache performance issue? I've been running varnishlog in all sorts of ways
trying to see any patterns in what requests are resulting in cache hits vs.
misses but no real luck there, either.
On Wed, Mar 23, 2016 at 11:19 AM, Justin Lloyd <jlloyd.wiki(a)gmail.com>
I recently upgraded my wikis from 1.24.2 to
1.26.2 and I'm trying to
diagnose why my Varnish hit ratios (4 web servers) dropped from ~85% to
~37%. Nothing in my Varnish configuration or VCL changed. For what it's
worth, I allocate 4 GB to Varnish. I'm fairly sure it has to do with
image/thumbnail caching, since there's about 30 GB worth of thumbnails
across all five wikis, though 12 GB of that is for our biggest and most
heavily trafficked wiki. (FWIW, I did change vcl_recv() last year to strip
cookies from thumbnail images and that improved overall performance while
maintaining a high hit ratio.) The Varnish n_lru_nuked stat is growing
rapidly as well, so it's definitely using all of the memory and having to
make room for new stuff.
Any thoughts on what might be causing this degradation, or at least how
to diagnose it?