No end to swap zone exhausted problems?

Anders Nordby anders at
Fri Oct 2 20:45:29 UTC 2009


Better post about my swap zone problems here than tear all my hair out.
This concerns me:

1) How can SWAPMETA usage continue increasing, while swap space used is
not? Example: In the swap meta
graph, used space is USED and free space is LIMIT-USED (FREE numbers are
a bit odd, it starts with 0) from the SWAPMETA line when running vmstat
-z. In the week graph, you can see that after 3 days swap space usage
settles at around 90 GB, while swap meta keeps increasing.

Is there a leak?

2) By default, kern.maxswzone is set to 32 MB in FreeBSD:

kern.maxswzone: 33554432

This leads to vmstat -z showing:

SWAPMETA: 288,   116519,        0,        0,        0, 0

According to vmstat man page, memory numbers are printed in kilobytes.
Where is the relation between maxswzone and SWAPMETA?

By booting with different settings for maxswzone and checking what
SWAPMETA LIMIT turns out to be, I get this table:

kern.maxswzone		SWAPMETA LIMIT
==============		==============

32 MB			116519

64 MB			233025

128 MB			466037

256 MB			932074

512 MB			995410

1 GB			995410

2 GB			995410

How come increasing maxswzone beyond 256 MB does not yield a larger
SWAPMETA than 995410? Does it stop around 260 MB maxswzone? How can I
increase SWAPMETA further to be able to swap more?

When maxswzone is set to 2 GB, the number goes negative:

kern.maxswzone: -2147483648

I guess my luck is out?

Some additional info:

- When LIMIT-USED from SWAPMETA gets to 0, the system freezes with the
dreaded "swap zone exhausted, increase kern.maxswzone" message. I
monitor this. It's nice to know when your system will freeze.

- KMEM settings are:

vm.kmem_size_max: 3865468109
vm.kmem_size_min: 0
vm.kmem_size: 2718302208

Had no luck by tuning and increasing kmem_size and kmem_size_max, it
mostly leads to crashes.

- I am running FreeBSD/amd64 7.2-RELEASE, and I have 8 GB physical RAM.

- By lowering kern.maxusers from 386 to 16 and trimming my kernel config
down to a bare minimum I got SWAPMETA LIMIT increased to a whopping
995475, an increase of ~64 KB. I need more than this.

- Why do I use so much swap space? I am running Varnish with malloc for
storage, and I have a rather large dataset (6 million objects for this
set of servers) with mostly 1 week expire time. Performance wise it
works well, better than mmapping a large file which is/was the most
common method for Varnish (this gives occasional ~2 minute freezes).



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