[Beowulf] physical memory

Hearns, John john.hearns at mclaren.com
Wed Apr 24 07:44:50 PDT 2013

> Does linux recombine physical memory into contiguous regions?

> My impression has been "no".  Somewhere down in the guts of the kernel there is the "slab" allocator, which maintains a data structure of free memory in power-of-two sizes.  As memory > is used, the chunks get broken up and naturally migrate towards the small, fragmented end of the allocator.

Interestingly enough, I have just been looking at this, not directly for HPC tasks but in connection with a backup software package which hit a bug.

Look at   cat /proc/buddyinfo

If your kernel has the CONFIG_COMPACTION flag set (but it does need to be compiled with that flag) You can trigger a memory decompaction by

echo 1 > /proc/sys/vm/compact_memory
(Oh Dear Lord - WHY have I referenced echoing ANYTHING to /proc/sys/vm again.)

And, yes, if you do a drop cache it does have an effect of the fragmentation as evidenced in /proc/buddyinfo

Mark Hahn will love me for that comment....

Ps. If I have misunderstood the original poster's comments, forgive me.
I have had my head stuck in  slabtop  /proc/meminfo and /proc/buddyinfo and examining  over the past few days to look at this issue,
I may have become somewhat deranged by it.

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