[Beowulf] physical memory
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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