[mapserver-users] MapCache - A way to reduce BerkeleyDB Cache size ?

[Andrew Cowie]

Hi Thomas

So size looks really good. Recreated one of our caches which has minimal 'blank tiles', and av. tile size is reasonably heavy ~ 24K

File Based:                                             65G
Previous BDB (empty-tiles branch)                       142G
BDB (empty-tiles branch) with PAGESIZE 1*1024           62G

However I have run into an issue with apache. Getting seg fault messages in apache error log, e.g.
[notice] child pid 5555 exit signal Segmentation fault (11)

Which requires restart of apache to make things work again. Was trying to work out the steps required to reproduce, restarting apache each time and I ran into a different error, I think the one you have mentioned before, relating to multiple restarts of apache

unable to allocate memory for mutex; resize mutex region
[Thu May 31 17:13:32 2012] [error] [client <IP>] failed to aquire connection to bdb backend: bdb cache failure 1 for db->open: Cannot allocate memory, referer: http:/server/bdb_test.html

Running a db_recover on it fixes this, as you mentioned.

I need to head home now, but happy to debug tomorrow, if you run me through the steps.

Cheers

Andrew



-----Original Message-----
From: thomas bonfort [mailto:thomas.bonfort at gmail.com]
Sent: Tuesday, 29 May 2012 11:07 p.m.
To: Ludovic Gnemmi; Andrew Cowie
Cc: mapserver-users at lists.osgeo.org
Subject: Re: [mapserver-users] MapCache - A way to reduce BerkeleyDB Cache size ?

Salut Ludovic, hi Andrew,

I've investigated a bit more on the disk space issue with berkeley db backends, and here are some of my findings. To summarize, the space occupancy/efficiency  is closely tied to the size of the images stored in the cache, and the configured pagesize of the bdb backend.

For a cache that contains many empty tiles, i.e. that are just a few hundred to a couple thousand bytes, here are the space occupancies (the disk cache uses 4k blocks). This is for reference only, usual caches will not resemble this one. Also note that the disk cache was not configured to symlink blank tiles, and that the bdb cache backend has some code in it to reduce the tile data stored on empty tiles.

disk: 197M
bdb pagesize=64k: 106M
bdb pagesize=4k: 96M
bdb pagesize=1k: 73M
sqlite: 103M
mbtiles (sqlite with blank tile detection): 62M

So these numbers actually aren't so bad for bdb in all configurations, but in this case the 4k filesytem blocksize is really inefficient.

These are the occupancies for another tileset, where the tiles are much heavier (tiles around 40-45k). There are also quite a few empty tiles.

disk:153M
bdb pagesize=64k: 209M
bdb pagesize=4k: 148M
bdb pagesize=1k: 144M
sqlite: 140M
mbtiles (sqlite with blank tile detection): 137M


I had initally stayed away from using small pagesizes in the bdb backend because of some stability issues, but in the current tests I have not run into any issues. From these tests, I think the natural way forward is to change the default pagesize, and maybe make it configurable in mapcache.xml as an advanced configuration option.
If you are able to confirm my findings on your instances, that would be great. To set the pagesize you'll have to recompile/reinstall, after having changed lib/cache_bdb.c by replacing

#define PAGESIZE 64*1024
by
#define PAGESIZE 1*1024

near line 50.

regards,
thomas



On Wed, May 23, 2012 at 3:55 PM, Ludovic Gnemmi <lgnemmi at rgd73-74.fr> wrote:
> Hi list,
> I would like to experiment Berkeley DB cache type but i'm surprised of
> the resulting db size compared to sqlite cache type. Here is my
> results for about 13000 tiles:
>
> Total tiles size  = 44 Mb
> Sqlite = 48 Mb
> Berkeley DB = 61 Mb
> Disk = 66 Mb (with 4kb filesystem block size)
>
> I thought i could reduce storage capacity with Berkeley DB cache type
> as with Sqlite but it seems there is not much difference with disk cache type.
> Is there a way to reduce Berkeley DB cache size ?
>
> Thanks for your help
>
> Ludovic
>
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