[Gluster-devel] brick multiplexing and memory consumption

[Jeff Darcy]

On Tue, Jun 20, 2017, at 03:38 PM, Raghavendra Talur wrote:
> Each process takes 795MB of virtual memory and resident memory is
> 10MB each.
Wow, that's even better than I thought.  I was seeing about a 3x
difference per brick (plus the fixed cost of a brick process) during
development.  Your numbers suggest more than 10x.  Almost makes it seem
worth the effort.  ;)
> Just to be clear, I am not saying that brick multiplexing isn't
> working. The aim is to prevent the glusterfsd process from getting
> OOM killed because 200 bricks when multiplexed consume 20GB of
> virtual memory.
Yes, the OOM killer is more dangerous with multiplexing.  It likes to
take out the process that is the whole machine's reason for existence,
which is pretty darn dumb.  Perhaps we should use oom_adj/OOM_DISABLE to
make it a bit less dumb?
> If it is found that the additional usage of 75MB of virtual memory per
> every brick attach can't be removed/reduced, then the only solution
> would be to fix issue 151 [1] by limiting multiplexed bricks.> [1] https://github.com/gluster/glusterfs/issues/151

This is another reason why limiting the number of brick processes is
preferable to limiting the number of bricks per process.  When we limit
bricks per process and wait until one is "full" before starting another,
then that first brick process remains a prime target for the OOM killer.
By "striping" bricks across N processes (where N ~= number of cores),
none of them become targets until we're approaching our system-wide
brick limit anyway.
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