Varnish Score Board, a tool to monitor threads

For a long time Varnish developers wanted a monitoring tool with a feature set similar to Apache’s mod_status, which lets you monitor the status of threads. Varnish is already well-served by several other, similar utilities: varnishstat, which shows internal counters, and varnishlog, which stores session information related to serving a request. 

One of the initial requirements was to ensure that this new tool avoided duplicating the information these existing Varnish tools provide. 


Why Varnish Score Board?

When we put together the Varnish Score Board, we envisioned a tool that would provide an overview of how a server is performing, and moreover, thought it would be handy for debugging and looking up hanging threads. And for now, for its initial release, that’s pretty much what it does.

Screen Shot 2019-10-09 at 17.17.20

The varnishscoreboard utility displays thread statuses from a running varnishd(1) instance. 

Take a look at an example output, displaying each active thread’s status in the columns and a summary of the number of threads running/in pool/max on the bottom line:

Age     Type     State        VXID     sesVXID     IP:Port                 Request

11.92s  CLIENT   FETCH        5505038  1079246861  xxx.xxx.xxx.xxx:51072   /index.html

1.80s   BACKEND  STARTFETCH   5308435  1079050258  xxx.xxx.xxx.xxx:51234   /login.html

1.85s   BACKEND  STARTFETCH   5013521  1078755344  xxx.xxx.xxx.xxx:51150   /pictures.html

6.92s   BACKEND  STARTFETCH   4030567  1077772390  xxx.xxx.xxx.xxx:50004   /index.html

...

11.88s  CLIENT   FETCH        6455320  1080197143  xxx.xxx.xxx.xxx:51096   /news.html

11.87s  CLIENT   DELIVER      6684686  1080426509  xxx.xxx.xxx.xxx:51100   /pictures.html

1.88s   BACKEND  STARTFETCH   6717463  1080459286  xxx.xxx.xxx.xxx:51084   /index.html

Threads running/in pool/max: 42/200/10000

varnishscoreboard allows you to monitor which resource is currently requested, and alongside VXID and varnishlog, you can easily get more information.

Under the hood, varnishd reserves a number of slots, equal to the number of threads configured, in the Varnish Shared Memory, and each new thread requests a slot at creation and writes updates on its status into it. With the default value of thread_pools = 2 and thread_pool_max = 5000, this is an extra ~2.85M.

Score board Diagram - Google Slides 22-10-2019 14-03-20

Since there is no locking involved at writing and reading, no performance penalties are introduced.

As this is a new feature, and it changes the default varnishd behavior by allocating more memory, it is disabled by default by the parameter: scoreboard_enable. After enabling the parameter, varnishd needs to be restarted. 

Another parameter, scoreboard_active can allow or forbid threads from requesting a slot to update their statuses. It requires scoreboard_enable to be on.

The feature is part of the varnish-plus-6.0.4r3 release. 


Tell us what you think

This tool is new, and as stated above, is designed to aid in debugging and looking up hanging threads. But what other applications could it have? How would you like to use varnishscoreboard? Do you have feature requests that could improve the tool’s usefulness? We'd love to hear your thoughts in the comments section below.

Topics: Varnish tips and tricks, varnish enterprise

21/10/2019, 15:30 by Dénes Mátételki

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