Some odd behaviour of vmstat and vmtotal...

[Alfred Perlstein]

* gnn at freebsd.org <gnn at freebsd.org> [080430 18:13] wrote:
> Howdy,
> 
> In deploying 7.0 at work we were finding a persistent problem when
> running "vmstat 1" on systems.  The problem shows up as a 10ms "pause"
> in processing, usually packet stamping and forwarding by a user level
> process.  This is due to the fact that vmstat calls the vmtotal()
> routine which in turn does a whole lot of locking.  The vmtotal call
> locks and walks the VM object list twice in a mark and sweep
> operation.
> 
> So, the question is, "What is the right way to fix this?"  I could
> remove the locking since the O_ACTIVE bit is not used by any other
> routine besides vmtotal, but I'm not too happy about that.
> 
> The relevant code can be found in src/sys/vm/vm_meter.c in vmtotal().
> 
> Thoughts?

Can you _really_ remove the locking?  I don't think that would be safe
if the list is being manipulated, but I haven't checked.

Typically one uses a sentinal to get around such problems.  Basically
insert a dummy object that no one should touch other than you, then
drop the lock periodically and insert it into the list, then resume.

-- 
- Alfred Perlstein