[Systems] Sunjammer Swapping

Bernie Innocenti bernie at codewiz.org
Fri Jan 1 03:20:02 EST 2016

My guess is that an unusual event occurred in the past (perhaps months
ago) caused the system to swap out a lot of stuff.

After the errant process got OOM-killed, some of the memory got swapped
back in, leaving in swap just the pages that were not referenced ever since.

There should be a munin graph showing swap I/O... and if it stays flat,
we're good.

On 01/01/2016 12:33 AM, James Cameron wrote:
> Sunjammer has 12 GB of memory, and you're talking about 3.3% of it.
> The "free -m" shows swap usage at 407 MB, which is 407 MB allocated by
> processes that almost never reference it.  (Referencing it will
> swap-in).
> So as it stands now, that's 407 MB of memory that the system is
> efficiently using for filesystem caching.
> I would leave swappiness as it is, or even increase it.
> However, you haven't shown the other side of the story; you've only
> looked at the static data.  What is the swap-in rate?  "vmstat" will
> show you the swap-in rate under the column "swap si".
> Where the long term swap-in rate is zero, the swap is costing you
> nothing, but is saving you physical memory.
> It is like having more memory.
> Yes, swap is slower than physical memory, but only when "swap-in" and
> "swap-out" are non-zero.  There's no cost to swapped unreferenced data.
> On Thu, Dec 31, 2015 at 11:05:59AM -0300, Samuel Cantero wrote:
>> Hello guys,
>> I was checking Sunjammer and I found the currently we are swapping around 407
>> MB but we have plenty of physical memory free yet.
>> Free -m shows:
>>                   total       used         free     shared    buffers    
>> cached
>> Mem:         11992     11673        319          0       1539       5981
>> -/+ buffers/cache:      4151       7841
>> Swap:         8191         407       7784
>> According to this, we have around 7841 MB free (~ 319 + 1539 + 5981). We must
>> consider that buffers and cache memory are unused memory that Linux borrows for
>> disk caching (for improve performance). But at the end this is free memory.
>> Applications can dispose of it immediately.
>> So what are we swapping? The answer is the swappiness value. This is
>> a tweakable setting that controls how often the swap file is used. A swappiness
>> setting of zero means that the disk will be avoided unless absolutely necessary
>> (we run out of memory), while a swappiness setting of 100 means that programs
>> will be swapped to disk almost instantly.
>> By default, Ubuntu comes with a default value of 60 (cat /proc/sys/vm/
>> swappiness). This means that the swap memory will be used quite often is the
>> memory usage is over the half usage. I consider that we should lower this value
>> to 10. With this setting, the swap memory will be only used when the RAM usage
>> is around 90%. We want to avoid disk usage as long as possible.
>> Suggestions and comments are welcome.
>> Best regards,
>> Samuel C.
>> _______________________________________________
>> Systems mailing list
>> Systems at lists.sugarlabs.org
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 _ // Bernie Innocenti
 \X/  http://codewiz.org

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