[mapserver-users] Slow and degrading performance with radar (but not satellite) on mapserver-6.0.1-3_0.el6.x86_64

[English Paul]

Hi,
   Newbie here, just got a work assignment to look into some performance issues with mapserver-6.0.1-3_0.el6.x86_64 – specifically, at one time, it rendered radar images very fast, then it degraded and seemed to consume a lot more CPU. So – the EC2 instance side was upgraded significantly – and it still performs badly and uses a lot of CPU. It also seems to be getting slowly worse over time (days/months, not seconds/minutes). I asked this on IRC, but it looks like email might be a better route.

  During all of this, the same mapserver instance renders satellite images quickly. These seem to be a similar, or in some cases larger size png to start with, rendered onto the same final map/size.

  So – the obvious answer is that it isn’t using the CPU to render/re-render, but rather spending it on something else – I/O most likely – eg: a network request, disk I/O, SQL query?

  The previous person working on it tried turning on debug at various levels – but unfortunately that made it even slower, making it tricky to answer “what is making it slow when debug is turned off?”

 My first instinct was to try an strace and nothing was obvious. Next up – a flame graph from strace, and/or trying dtrace – but my understanding is that dtrace is a little weak on RHEL/CentOS/Amazon Linux 6.0 *and* I’m not particularly good at that. Also, we’re using fcgi, so attaching to the correct process is a bit tricky.

  My next instinct was to look at release notes and see if the current stable has anything fixes/improvements that directly address this – there aren’t any that are obvious to my eyes, but you developers have been busy! So many things! Including some performance fixes and one “significant” performance fix.

Current config file:

AddHandler fcgid-script fcgi

FcgidIPCDir /var/run/mod_fcgid
FcgidProcessTableFile /var/run/mod_fcgid/fcgid_shm
FcgidMaxProcesses 10
FcgidMaxProcessesPerClass 10
FcgidMaxRequestInMem 196608
FcgidInitialEnv PROJ_LIB /usr/share/proj
FcgidInitialEnv LD_LIBRARY_PATH "/usr/local/lib:/usr/pgsql-9.1/lib"

So – suggestions for my next move? I currently plan to take a quick swing at building 7.6 for RHEL 6.0, knowing there might be old libraries and whatnot that make that a non-starter. Of course – we’ve got other infra running on this same instance, so upgrading everything is a much bigger task.

 Strace flame graph?

  Stretch and try dtrace?

  A better way to use debug?

  Something else I’m missing – eg: differences between the image types that make them perform so differently?

Thanks,
Paul
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