doc: refer to malloc-related environment variables

Using non-portable mallopt/mallctl functions is not feasible because
detecting them correctly at _link_ time is not easy.  Detecting them
at compile time is insufficient because malloc implementations can
be swapped at link time (and even with LD_PRELOAD, unfortunately).

1 files changed, 16 insertions, 0 deletions

diff --git a/cmogstored.x b/cmogstored.x
index aa3167c..3fcc91b 100644
--- a/cmogstored.x
+++ b/cmogstored.x
@@ -15,6 +15,22 @@ MOG_IOSTAT_CMD - command-line for invoking iostat(1).  This is used
 by the sidechannel to report disk utilization to trackers.
 Default: "iostat -dx 1 30"
 
+See MALLOC TUNING for environment variables which may affect memory
+usage.
+
+[MALLOC TUNING]
+
+malloc(3) implementations tuned for multithreading speed (and not memory
+usage) may cause large amounts of metadata overhead in infrequently-used
+arenas.  While cmogstored may use many threads for filesystem I/O,
+cmogstored is designed to not be dependent on malloc(3) performance nor
+scalability.
+
+For on GNU/Linux systems with glibc, we recommend setting
+MALLOC_ARENA_TEST and MALLOC_ARENA_MAX to low numbers to limit the
+number arenas.  Setting the value of each to 1 or 2 should be enough,
+even.
+
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