[mapserver-users] Understanding MapServer Memory Usage with WMS raster layers

[lars.schylberg at blixtmail.se]

Hi,




Thanks for the good info Peter !!





Looking at the text just below these rows:




https://github.com/mapserver/mapserver/blob/66309eebb7ba0dc70469efeb40f865a8e88fafbd/mappool.c#L62-L65




I see that there is an option that is not documented:




CLOSE CONNECTION=NORMAL




Shouldn't that option be described in the manual page for LAYER, even if it is the default behavior  ?



/Lars Schylberg


-----Originalmeddelande-----
Från: "Peter Schmitt" <pschmitt at gmail.com>
Till: mapserver-users at lists.osgeo.org
Datum: 2017-10-27 20:58
Ämne: Re: [mapserver-users] Understanding MapServer Memory Usage with WMS raster layers

I think I understand what's going on now and want to share my findings for future Google searchers wondering the same thing.

I see unbounded memory growth with many raster layers having a single image because the default behavior of raster layers lacking a Tile Index is DEFER.
https://github.com/mapserver/mapserver/blob/93e3eb28b49b508dd610af2fde118f04e2177f13/mapraster.c#L838-L839



With DEFER, the Connection Pool will keep the rasters open until mapserv closes.
https://github.com/mapserver/mapserver/blob/66309eebb7ba0dc70469efeb40f865a8e88fafbd/mappool.c#L62-L65

My setup (https://github.com/pedros007/debian-mapserver Docker container with Supervisord running 8 mapserv processes routed via nginx fcgi) ensures mapserver never exits.  So with my Mapfile of many large single-file raster layers results in many large rasters which are opened indefinitely by 8 separate mapserv processes.


My typical Mapfile setup has many raster layers each with a tile index of many rasters. In this configuration, I don't see unbounded memory growth.  Looks like the default behavior with a tile index is to close each raster in each MapServer request! https://github.com/mapserver/mapserver/blob/93e3eb28b49b508dd610af2fde118f04e2177f13/mapraster.c#L845


In short, I can prevent unbounded memory growth by setting CLOSE_CONNECTION=ALWAYS on single-image raster layers.  I suppose I figured that out before writing my original question to the mailing list, but at least I now know why this solves my problem.  Sorry for adding noise to the list!





On Wed, Oct 25, 2017 at 11:42 AM, Peter Schmitt <pschmitt at gmail.com> wrote:
Hi,

I am using MapServer as a WMS endpoint serving rasters. My configuration has Supervisord running 8 mappserv processes which are listening on a socket and dispatched via Nginx FastCGI. This all runs in the following Docker image: https://github.com/pedros007/debian-mapserver


I noticed my memory utilization monotonically increases over time only drops when there's a log in dmesg on the host Linux OS (... /not/ in the OS of the Docker container) like the following:


      [508072.641250] Memory cgroup out of memory: Kill process 21147 (mapserv) score 143 or sacrifice child
      [508072.646072] Killed process 21147 (mapserv) total-vm:672384kB, anon-rss:449172kB, file-rss:0kB, shmem-rss:0kB


I can dramatically lower my memory footprint by adding this directive to every raster LAYER as suggested by  https://github.com/mapserver/mapserver/issues/4389


      PROCESSING "CLOSE_CONNECTION=ALWAYS"


With all of that, I have a few questions:


1.  With the default CLOSE_CONNECTION=DEFER on a raster layer, are raster file handles ever closed?
2.  What does the config variable GDAL_CACHEMAX do in relation to CLOSE_CONNECTION = DEFER or ALWAYS?  I assume with processing directive CLOSE_CONNECTION=ALWAYS, GDAL_CACHEMAX does nothing for me.
3.  My Mapfile actually reads rasters at /vsicurl/ URLs.  I assume that VSI_CACHE=TRUE and VSI_CACHE_SIZE are a LRU cache which is in addition to the GDAL cache and any OS/host level caching of open file handles. Is the same data cached separately in up to 3 places (GDAL cache, Host cache and VSI cache)?


Thanks!
Pete






--
Pete

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