Varnish, long lived cache and purge on change

[Ken Brownfield]

Hey Karl. :-)

The implementation of purge in Varnish is really a queue of refcounted  
ban objects.  Every image hit is compared to the ban list to see if  
the object in cache should be reloaded from a backend.

If you have purge_dups off, /every/ request to Varnish will regex  
against every single ban in the list.  If you have purge_dups on, it  
will at least not compare against duplicate bans.

However, a ban that has been created will stay around until /every/  
object that was in the cache at the time of that ban has been re- 
requested, dupe or no.  If you have lots of content, especially  
content that may not be accessed very often, the ban list can become  
enormous.  Even with purge_dups, duplicate ban entries remain in  
memory.  And the bans are only freed from RAM when their refcount hits  
0 /AND/ they're at the very tail end of the ban queue.

Because of the implementation, there's no clear way around this AFAICT.

You can get a list of bans with the "purge.list" management command,  
but if it's more than ~2400 long you'll need to use netcat to get the  
list.  Also, purged dups will NOT show up in this list, even though  
they're sitting on RAM.  I have a trivial patch that will make dups  
show up in purge.list if you'd like to get an idea of how many bans  
you have.

The implementation is actually really clever, IMHO, especially with  
regard to how it avoids locks, and there's really no other scalable  
way to implement a regex purge that I've been able to dream up.

The only memory-reducing option within the existing implementation is  
to actually delete/free duplicate bans from the list, and to delete/ 
free bans when an object hit causes the associated ban's refcount to  
hit 0.  However, this requires all access to the ban list to be  
locked, which is likely a significant performance hit.  I've written  
this patch, and it works, but I haven't put significant load on it.

I'm not sure if Varnish supports non-regex/non-wildcard purges?  This  
would at least not have to go through the ban system,  but obviously  
it doesn't work for arbitrary path purges.

We version our static content, which avoids cache thrash and this  
purge side-effect.  This is very easy if you have a central URL- 
generation system in code (templates, ajax, etc), but probably more  
problematic in situations where the URL needs to be "pretty".

Ken

On Aug 18, 2009, at 4:06 PM, Karl Pietri wrote:

> Hello everyone,
>     Recently we decided that our primary page that everyone views  
> doesn't really change all that often.  In fact it changes very  
> rarely except for the stats counters (views, downloads, etc).  So we  
> decided that we wanted to store everything in varnish for a super  
> long time (and tell the client its not cacheable or cacheable for a  
> very short amount of time), flush the page from varnish when it  
> truly changes and have a very fast ajax call to update the stats.   
> This worked great for about 2 days.   Then we ran out of ram and  
> varnish started causing a ton of swap activity and it increased the  
> response times of everything on the site to unusable.
>
> After poking about i seem to have found the culprit.  When you use  
> url.purge it seems to keep a record of that and check every object  
> as it is fetched to see if it was purged or not.  To test this i set  
> a script to purge a lot of stuff and got the same problem to happen.
>
>
> from varnishstat -1
>
>  n_purge                236369          .   N total active purges
> n_purge_add            236388         2.31 N new purges added
> n_purge_retire             19         0.00 N old purges deleted
> n_purge_obj_test      1651452        16.12 N objects tested
> n_purge_re_test    5052057513     49316.27 N regexps tested against
> n_purge_dups                0         0.00 N duplicate purges removed
>
> each uptick is when i add 100k new purge records.  you can see what  
> will happen soon.
>
> http://wrenchies.net/~karl/Automator/ScreenShots/20090818160328.png
>
> We really want to take advantage of this style of essentially having  
> static html served by varnish and flush it out when it changes.
>
> Does any one have advice on how to do this?
>
> Originally we had implemented this using the vlc to set the ttl to 0  
> but with all the combinations of accept-encoding that are possible  
> we were getting many things not being purged from the cache.
>
> Another thought would be to refetch the page on change instead of  
> purging it but that has the same problem with accept-encoding.
>
> after 36 hours between the 2 machines we have collected 1.3M objects  
> in the cache and have not even come close to running out of space.   
> We would actually like to increase our ttl for the cached objects  
> even longer.
>
> I hope someone can help me out here.
>
> -Karl Pietri
> _______________________________________________
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> varnish-misc at projects.linpro.no
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