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List:       varnish-misc
Subject:    RE: High CPU usage when using ban with HTTP PURGE
From:       Connor Walls <connor.walls () skillpages ! com>
Date:       2012-07-30 11:04:51
Message-ID: B9FE204AB6292842A9B015B87DD4CFA74AD3135D55 () SBS-SERVER ! weedle ! local
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Hi Lasse, thanks for the prompt reply. I've done a little bit more investigation on \
this issue.

1. I tried using smart bans but I saw no change in performance. I tried change the \
ban from a regex to a straight match on the URL (i.e. from ban("obj.http.x-url ~ " + \
[header containing regex]) to ban("obj.http.x-url == " + [header containing url]) ) \
and I still observed the same performance issues. I also tried changing from ban() to \
ban_url() - no performance improvement. Removing that line from the VCL brings the \
CPU right back down, so I've concluded the problem is adding the ban to the ban list. \
Is this an inherently CPU intensive operation, is there going to be no way around \
this?

2. I've also gone down the route of trying purges because they are significantly \
faster and look like they can handle the load okay, but my only issue is matching \
variations of a page. My vcl_hash function looks like this:

sub vcl_hash {
   hash_data(req.http.Accept);
   hash_data(req.http.X-Forwarded-Proto);
}

So the problem I'm having is purging all variations of the file based on "Accept" and \
"X-Forwarded-Proto" headers, as well as the host. Is this something that is even \
possible? I personally wouldn't have thought so but after seeing in the documentation \
(https://www.varnish-cache.org/docs/trunk/tutorial/purging.html):  "The purge in \
vcl_miss is necessary to purge all variants in the cases where you hit an object, but \
miss a particular variant." Seems to imply that this functionality is present but I \
can't find any more documentation on how it works. Do I need to remove the \
hash_data() calls in vcl_hash and instead include "Accept" and "X-Forwarded-Proto" in \
the "Vary" header coming from the backend server?

Thanks,
Connor Walls

-----Original Message-----
From: varnish-misc-bounces@varnish-cache.org \
                [mailto:varnish-misc-bounces@varnish-cache.org] On Behalf Of Lasse \
                Karstensen
Sent: 26 July 2012 10:36
To: varnish-misc@varnish-cache.org
Subject: Re: High CPU usage when using ban with HTTP PURGE

Connor Walls:
> So, I've been having an issue with CPU usage recently. We run a 
> cluster of
> 3 varnish boxes, running 3.0.2, serving content on a particular port, 
> and accepting HTTP PURGE request on another (:82). This is used so 
> whenever a user on the site changes something on their profile page, 
> one of the backend servers will send a HTTP PURGE request to each box 
> in the cluster, containing a regex in the header, which is then used 
> to ban based on a particular URL - the relevant VCL is as follows:
[..]
> Now what we've been seeing is very high CPU usage with this 
> functionality, which has been slowly growing over time with increased 
> traffic on the website, and is now averaging above 70%, peaking at 
> 100% at times and causing some user requests to fail. The CPU usage 
> correlates very closely with timeout exceptions we get when making the 
> HTTP PURGE requests. This became an issue yesterday to the point where 
> we disabled the HTTP PURGE - CPU usage has now fallen from averaging > 
> 70% to averaging around 8%, so clearly the purges are the issue. What 
> I'm wondering is whether or not banning like this is going to be 
> inherently heavy and I need to find a way to throttle the purge 
> requests, or if I could rework the VCL so it will be less CPU heavy to 
> process these requests? Will the CPU usage depend on what the actual regex is?

There are two methods for cache invalidation in Varnish 3.0: Purges and bans.
Bans are flexible and cool, and purges are less flexible but very fast. Use purges if \
you can.

If you are using a high amount of bans, you should use what Kristian calls "Smart \
bans" in the Varnish Book: 

    https://www.varnish-software.com/static/book/Cache_invalidation.html#smart-bans


--
Lasse Karstensen
Varnish Software AS
http://www.varnish-software.com/

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