Bug 22840 - Mono Boehm GC is limited to allocating 64GB
Summary: Mono Boehm GC is limited to allocating 64GB
Alias: None
Product: Runtime
Classification: Mono
Component: GC ()
Version: 3.4.0
Hardware: PC Linux
: --- normal
Target Milestone: ---
Assignee: Bugzilla
Depends on:
Reported: 2014-09-10 13:43 UTC by Craig Minihan
Modified: 2014-10-03 09:36 UTC (History)
4 users (show)

Is this bug a regression?: ---
Last known good build:

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Description Craig Minihan 2014-09-10 13:43:15 UTC
In libgc/include/private/gc_priv.h there is the following define:

#   if CPP_WORDSZ > 32
#     define MAX_HEAP_SECTS 4096 	/* overflows at roughly 64 GB	   */ <=== here is the problem!
#   else
#     define MAX_HEAP_SECTS 768		/* Separately added heap sections. */
#   endif
# else
#   ifdef SMALL_CONFIG
#     define MAX_HEAP_SECTS 128		/* Roughly 256MB (128*2048*1K)	*/
#   else
#     define MAX_HEAP_SECTS (384+128)		/* Roughly 4GB			*/
#   endif
# endif

My application uses more than 64GB so I hit this case in libgc/alloc.c: 
if (GC_n_heap_sects >= MAX_HEAP_SECTS) {
    ABORT("Too many heap sections: Increase MAXHINCR or MAX_HEAP_SECTS");

I build from source so I can change this define value. However it would be nice to increase this limit for binary installs of Mono too.

A new limit of 1TB or higher would be great.
Comment 1 Craig Minihan 2014-09-11 06:28:09 UTC
I updated MAX_HEAP_SECTS to 16384 locally and have my Mono process (3.4) running quite happily now with 150GB allocated.
Comment 2 Craig Minihan 2014-09-26 05:37:17 UTC
This issue is for Boehm only - not sgen.

On CentOS sgen required the following setting change:
$ sysctl vm.max_map_count=655360

The default value of 65530 doesn't work above ~30GB or so.
Comment 3 Rodrigo Kumpera 2014-10-03 08:26:48 UTC
We have no plans to change Boehm any further.
Comment 4 Craig Minihan 2014-10-03 09:36:09 UTC
That's a shame since 64GB+ servers can be rented for ~$100 a month - not exactly a high end configuration. Boehm also seems more stable than sgen at v high memory load.