intr using Swap

Lowell Gilbert freebsd-stable-local at be-well.ilk.org
Thu Feb 18 00:50:48 UTC 2016


hiren panchasara <hiren at strugglingcoder.info> writes:

> On 02/17/16 at 04:44P, Efra?n D?ctor wrote:
>> El 17/02/2016 a las 01:15 p. m., dweimer escribi?:
>> >
>> > They may not show as swapped unless the entire process is actually 
>> > swapped, which would be unlikely to occur. Personally I wouldn't worry 
>> > about it, the only thing I can think of is to restart processes one at 
>> > a time to see which one clears up the swap usage. Granted you may see 
>> > a little clear after each process.
>> >
>> > The more important task would be to determine what caused the memory 
>> > to run out in the first place, and decide if its going to be a 
>> > frequent enough occurrence to justify adding physical memory to the 
>> > system.
>> >
>> > There is likely some way to find out what is using it, but that is 
>> > beyond my knowledge.
>> >
>> > -- 
>> > Thanks,
>> >    Dean E. Weimer
>> > http://www.dweimer.net/
>> 
>> The server has 64 GB of RAM, 40-45 GB are always inactive thats why I'm 
>> wondering why are the processes being swapped out.

There are almost certainly no processes being swapped out. Some of their
*pages* are being stored in swap, but that is a very different thing.

> Yes, I've seen this too. Inact end up accumulating a very large chunk of
> memory leaving Free to very low. 

That's yet another, different thing.

> What VM/pagedaemon seems to care about is Free+Cache and not just Free.

Which makes sense, even without a deep understanding of the concepts,
because those are guaranteed to be able to be re-allocated immediately.
It is literally true that the pageout scan does nothing when free+cache
is less than the target.

> I kind of get that Free mem is wasted mem but putting everything in Inact
> to the point that machine has to go into swap when a sudden need arises
> also doesn't seem right.

"Go into swap" is too vague to mean much.  I suspect that you mean the
system will have to start swapping out rapidly, but that isn't actually
the case. First of all, pages in "inact" aren't necessarily dirty, so
re-using them wouldn't be as expensive as having to write them to
backing store. Also, when a page is copied to swap, the surrounding
pages are eligible to be copied to swap also, to take advantage of the
bandwidth advantages of larger transfer sizes. This does not move them
to the cache queue, although it does make that easier to do later if and
when their "turn" comes up.

> I guess it all boils down to adjusting defaults to the system's need.
> i.e. if you know you have a proc that may need a large chunk of mem
> you'd need to tweak free+cache target accordingly. What I find lacking
> is the correct/easy way to do it. If I look at available sysctls:
> vm.v_free_min: Minimum low-free-pages threshold
> vm.v_cache_min: Min pages on cache queue
> vm.v_free_target: Desired free pages
> And I cannot get them to do the right thing to have more Free around so
> swapping doesn't happen in sudden need. And are these all runtime
> sysctls? OR does it require reboot for them to work right? 

These take effect immediately, from what I can see.

Have you measured that paging (not swapping; that's a more extreme
measure where the whole process gets removed from memory) is a
significant load on your system in a specific case? If not, I doubt that
it's actually the case, and you're mitigating a non-existent problem.

Be well.


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