FreeBSD using swap even though there's a lot of free memory
Twingly Customer Support
team at twingly.com
Mon Oct 19 14:06:32 UTC 2020
> Are you using a swap backed tmpfs ?
No, we don't use tmpfs. This is how we define our swap partition:
% cat /etc/fstab
# Device Mountpoint FStype Options Dump Pass#
/dev/mirror/swap.eli none swap sw 0 0
Fortunately the problem does no longer occur since this Friday, Oct 16, which is when we ran "pkg upgrade -y" the last time. There has been no swap usage (or laundry either for that matter) since then, also the memory (active + inactive + wired) has now been over 200G for the first time since the problems started. I have started ZFS scrub, and even after that the swap is empty.
We update packages regularly, and we have done that multiple times since our problems started, but this time the problem went away. We have not made any other changes to the system recently (other than updating packages) that could have caused these problems. I have not taken a closer look at what exactly has changed in the packages that got updated yet, but these are all the packages that was changed (output from "pkg upgrade -y"):
Installed packages to be UPGRADED:
gnutls: 3.6.13 -> 3.6.14
libnghttp2: 1.40.0 -> 1.41.0
perl5: 5.30.2 -> 5.30.3
python37: 3.7.7 -> 3.7.7_1
I'll continue and see if I manage to figure out which (if any) of the above packages could have caused this.
Also, thanks to Doug for the following:
> I encountered this issue a year or so ago. In my case it turned out to =
> be a process that was allocating anonymous segments using mmap. It =
> tended to forget to free them after the usage was complete. As a =
> result, the system would run out of swap even though there was plenty of =
> free memory. Anonymous segments are mapped to swap space. I traced the =
> problem using:
>
> procstat -va | grep df
>
> That generated a lot of data. I looked for processes that had an =
> increasing number of df files. Eventually I found the right one and =
> fixed it.
I'll be sure to remember this in case I run into things like these in the future.
// Mattias
--------------------------------------------------------------------------------
On October 16, 2020 20:57 Steve O'Hara-Smith wrote:
On Thu, 15 Oct 2020 15:23:51 +0100
Twingly Customer Support <team at twingly.com> wrote:
> Eventually, after scrubbing a few times, the swap becomes full and we
> start seeing "swap_pager_getswapspace(24): failed" etc. in dmesg.
Are you using a swap backed tmpfs ?
--
Steve O'Hara-Smith <steve at sohara.org>
--------------------------------------------------------------------------------
On October 16, 2020 19:50 doug wrote:
On Thu, 15 Oct 2020, Jon Schneider wrote:
>top -w -oswap
>
>seems to report the right thing. Would be interesting if it's _not_ MySQL.
>
>Jon
>
>On 15/10/2020 15:23, Twingly Customer Support wrote:
>>Hi,
>>
>>We have a server running FreeBSD 12.1-RELEASE-p10. We currently have a problem where FreeBSD starting to swap when running ZFS scrub, even though we have ~70G of free memory.
>>This did not happen before when running FreeBSD 11.3 for example. It started happening at approximately the time we upgraded from 12.1-RELEASE-p5 to 12.1-RELEASE-p6, but if the upgrade is the cause of the problem is unclear, though FreeBSD never swapped for us before that. "Laundry" memory was not something we saw before either, it started to appear at the same time as FreeBSD started swapping.
>>
>>Eventually, after scrubbing a few times, the swap becomes full and we start seeing "swap_pager_getswapspace(24): failed" etc. in dmesg.
>>This is the memory usage a while after scrubbing, note the values for Mem/Free and Swap:
>>
>>```
>>% top | head -n 7
>>last pid: 8112; load averages: 1.82, 1.77, 1.73 up 6+01:37:42 10:53:48
>>35 processes: 1 running, 34 sleeping
>>CPU: 4.9% user, 0.0% nice, 4.2% system, 0.2% interrupt, 90.7% idle
>>Mem: 110G Active, 27G Inact, 5413M Laundry, 39G Wired, 68G Free
>>ARC: 34G Total, 28G MFU, 4101M MRU, 53M Anon, 1317M Header, 225M Other
>> 30G Compressed, 53G Uncompressed, 1.77:1 Ratio
>>Swap: 8192M Total, 6434M Used, 1757M Free, 78% Inuse
>>```
>>
>>We are running MySQL, which has been configured to use ~50% of the total amount memory (using innodb_buffer_pool_size=127748M)
>>ZFS ARC has been configured to use 25% of the total memory (using vfs.zfs.arc_max="63874M")
>>
>>We have tried raising both vfs.zfs.arc_max and innodb_buffer_pool_size, but this did not make any change to the total memory usage, the free memory stays at around 70G and FreeBSD still started swapping.
>>It's as if the memory is capped at around 180G for some reason.
>>
>>Are there any configuration values that could cause FreeBSD to swap even though there's free memory? Are there any config values one could try to change in order to get FreeBSD to use the remaining ~70G of free memory instead of swapping?
>>
>>Let me know if there's any more details you want me to provide and I'll attach those.
>>
>>Thanks!
>>
>>// Mattias
I see similar things. The Jails in question are 11.1. The systems updated to 12.1 do not display this behavior. This 11.1 system runs 5 jails. Swapinfo is shown below:
Device 1K-blocks Used Avail Capacity
/dev/aacd0p3 4194304 1776000 2418304 42%
These numbers are developed from top on the base system
[ 0 50861 ] root
[ 20 281903 ] camden squirellmail/roundcube/postfix/mysql
[ 21 322759 ] bassharbor wordpress/php56
[ 19 343522 ] monhegan wordpress/php56
[ 18 369139 ] newharbor apache24/sendmail
[ 17 587332 ] pemaquid wordpress/php74
Jails: 1904655
total: 1955516
I read somewhere that the virtual memory system pre-pages modified pages as a just-in-case measure. If this is correct, 12.1 does not do this on a non-paging system. The system shown above uses about 10% swapspace after a reboot and works its way to the 42% shown above in a day or so.
--------------------------------------------------------------------------------
On October 15, 2020 18:21 Jon Schneider wrote:
top -w -oswap
seems to report the right thing. Would be interesting if it's _not_ MySQL.
Jon
--------------------------------------------------------------------------------
On October 15, 2020 16:24 Team Twingly wrote:
Hi,
We have a server running FreeBSD 12.1-RELEASE-p10. We currently have a problem where FreeBSD starting to swap when running ZFS scrub, even though we have ~70G of free memory.
This did not happen before when running FreeBSD 11.3 for example. It started happening at approximately the time we upgraded from 12.1-RELEASE-p5 to 12.1-RELEASE-p6, but if the upgrade is the cause of the problem is unclear, though FreeBSD never swapped for us before that. "Laundry" memory was not something we saw before either, it started to appear at the same time as FreeBSD started swapping.
Eventually, after scrubbing a few times, the swap becomes full and we start seeing "swap_pager_getswapspace(24): failed" etc. in dmesg.
This is the memory usage a while after scrubbing, note the values for Mem/Free and Swap:
```
% top | head -n 7
last pid: 8112; load averages: 1.82, 1.77, 1.73 up 6+01:37:42 10:53:48
35 processes: 1 running, 34 sleeping
CPU: 4.9% user, 0.0% nice, 4.2% system, 0.2% interrupt, 90.7% idle
Mem: 110G Active, 27G Inact, 5413M Laundry, 39G Wired, 68G Free
ARC: 34G Total, 28G MFU, 4101M MRU, 53M Anon, 1317M Header, 225M Other
30G Compressed, 53G Uncompressed, 1.77:1 Ratio
Swap: 8192M Total, 6434M Used, 1757M Free, 78% Inuse
```
We are running MySQL, which has been configured to use ~50% of the total amount memory (using innodb_buffer_pool_size=127748M)
ZFS ARC has been configured to use 25% of the total memory (using vfs.zfs.arc_max="63874M")
We have tried raising both vfs.zfs.arc_max and innodb_buffer_pool_size, but this did not make any change to the total memory usage, the free memory stays at around 70G and FreeBSD still started swapping.
It's as if the memory is capped at around 180G for some reason.
Are there any configuration values that could cause FreeBSD to swap even though there's free memory? Are there any config values one could try to change in order to get FreeBSD to use the remaining ~70G of free memory instead of swapping?
Let me know if there's any more details you want me to provide and I'll attach those.
Thanks!
// Mattias
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