On Mon, Dec 20, 2010 at 9:25 AM, Kristian Lyngstol < [email protected]> wrote:
> On Fri, Dec 17, 2010 at 03:40:35PM +0100, Stig Bakken wrote: > > I have a script processing varnishlog output to give me the serving > > latency based on the ReqEnd tag, and frequently see objects served from > > cache, size around 25kB, that take 5+ seconds to serve. > > If there are two requests for an object, request A and request B, the > following will happen: > > Time | Action > 0s | A asks for /foo > | Cache-miss - fetch from backend > 0.5s | B asks for /foo > | Cache hit on busy object - wait for it > 5s | Web server is finished delivering /foo > 5.1s | A gets /foo - cache miss > 5.1s | B gets /foo - cache hit > > It's a bit difficult to spot this as it happens, it's been a matter of > discussion recently (Arthur being the one who first became aware of the > issue). > > It sounds like this might be what you are seeing. > > - Kristian Yeah, that could be it. A race condition that is to be expected. I'll test whether this explains the issue by comparing 95/99-percentile latency with cache hit rate over time. - Stig -- Stig Bakken CTO, Zedge.net - free your phone!
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