* gnn@freebsd.org <gnn@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