Debug: On Waiting list?

[Poul-Henning Kamp]

In message <00fkbog5bxjat75.161120071108 at server2k7>, Erik writes:
>Hi,
>Im struggling with a couple of high response times when using
> varnish on our site. I have configured varnish so it will
> cache only static content (jpg,gif,png,css,js etc) even if a
> cookie is set. But all our jpeg's are getting a high response
> time when Im running some test against the site. 
>One of the jpeg's are pic_mini.jpg, it has a response time of 357ms. I turned on logging and vcl_trace and found the line:
>
>   14 Debug        c "Hash Match:
> /Portals/0/Picture/pic_mini.jpg#www3.mydomain.com#"
>   14 Debug        c "on waiting list on obj 1767384108"

On waiting list means that somebody else asked for that object from
the backend, and that this session now awaits that they get a
response before it continues.

If the other session gets a cacheable response, this session will
return that cached reply, but if the result is not cacheable,
this session will then have to go to the backend.

The idea here, is that when an object is not in the cache, we
should not send hordes of clients to the backend after it, if
the first one can do the job alone.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.