Varnish “sess_workspace” and why it is important

When using Varnish on a high traffic site like opera.com or my.opera.com, it is important to reach a stable and sane configuration (both VCL and general service tuning).

If you're just starting using Varnish now, it's easy to overlook things (like I did, for example :) and later experience some crashes or unexpected problems.

Of course, you should read the Varnish wiki, but I'd suggest you also read at least the following links. I found them to be very useful for me:

A couple of weeks ago, we experienced some random Varnish crashes, 1 per day on average. That happened during a weekend. As usual, we didn't really notice that Varnish was crashing until we looked at our Munin graphs. Once you know that Varnish is crashing, everything is easier :)

Just look at your syslog file. We did, and we found the following error message:

Feb 26 06:58:26 p26-01 varnishd[19110]: Child (27707) died signal=6
Feb 26 06:58:26 p26-01 varnishd[19110]: Child (27707) Panic message: Missing errorhandling code in HSH_Prepare(), cache_hash.c line 188:#012  Condition((p) != 0) not true.  thread = (cache-worker)sp = 0x7f8007c7f008 {#012  fd = 239, id = 239, xid = 1109462166,#012  client = 213.236.208.102:39798,#012  step = STP_LOOKUP,#012  handling = hash,#012  ws = 0x7f8007c7f078 { overflow#012    id = "sess",#012    {s,f,r,e} = {0x7f8007c7f808,,+16369,(nil),+16384},#012  },#012    worker = 0x7f82c94e9be0 {#012    },#012    vcl = {#012      srcname = {#012        "input",#012        "Default",#012        "/etc/varnish/accept-language.vcl",#012      },#012    },#012},#012
Feb 26 06:58:26 p26-01 varnishd[19110]: Child cleanup complete
Feb 26 06:58:26 p26-01 varnishd[19110]: child (3710) Started
Feb 26 06:58:26 p26-01 varnishd[19110]: Child (3710) said Closed fds: 3 4 5 10 11 13 14
Feb 26 06:58:26 p26-01 varnishd[19110]: Child (3710) said Child starts
Feb 26 06:58:26 p26-01 varnishd[19110]: Child (3710) said Ready
Feb 26 18:13:37 p26-01 varnishd[19110]: Child (7327) died signal=6
Feb 26 18:13:37 p26-01 varnishd[19110]: Child (7327) Panic message: Missing errorhandling code in HSH_Prepare(), cache_hash.c line 188:#012  Condition((p) != 0) not true.  thread = (cache-worker)sp = 0x7f8008e84008 {#012  fd = 248, id = 248, xid = 447481155,#012  client = 213.236.208.101:39963,#012  step = STP_LOOKUP,#012  handling = hash,#012  ws = 0x7f8008e84078 { overflow#012    id = "sess",#012    {s,f,r,e} = {0x7f8008e84808,,+16378,(nil),+16384},#012  },#012    worker = 0x7f81a4f5fbe0 {#012    },#012    vcl = {#012      srcname = {#012        "input",#012        "Default",#012        "/etc/varnish/accept-language.vcl",#012      },#012    },#012},#012
Feb 26 18:13:37 p26-01 varnishd[19110]: Child cleanup complete
Feb 26 18:13:37 p26-01 varnishd[19110]: child (30662) Started
Feb 26 18:13:37 p26-01 varnishd[19110]: Child (30662) said Closed fds: 3 4 5 10 11 13 14
Feb 26 18:13:37 p26-01 varnishd[19110]: Child (30662) said Child starts
Feb 26 18:13:37 p26-01 varnishd[19110]: Child (30662) said Ready

A quick research brought me to sess_workspace.

We found out we had to increase the default (16kb), especially since we're doing quite a bit of HTTP header copying and rewriting around. In fact, if you do that, each varnish thread uses a memory space at most sess_workspace bytes.

If you happen to need more space, maybe because clients are sending long HTTP header values, or because you are (like we do) writing lots of additional varnish-specific headers, then Varnish won't be able to allocate enough memory, and will just write the assert condition on syslog and drop the request.

So, we bumped sess_workspace to 256kb by setting the following in the startup file:


-p sess_workspace=262144

And since then we haven't been having crashes anymore.

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