warning: total configured swap (2621440 pages) exceeds maximum
recommended amount (1852656 pages).
...
This is because kernel needs some memory to manage swap too.
Currently for amd64 this roughly reduces to the following rule
(My apologies in advance for the extra simplification):
100MB RAM per 800MB swap space.
That is oversimplified to the point of being wrong. As of HEAD
r239255 and 9-stable r240097, there's no longer a limit on amd64. The
limit is still required on 32-bit architectures due to the limited KVA
available.
The original poster; i.e., me, copied the error message directly from
amd64 running r240891. Here is some information after /etc/fstab swap was
left untouched and zfs swap was reduced:
(11:14) dmesg | grep recommended
warning: total configured swap (1854464 pages) exceeds maximum recommended
amount (1852656 pages).
(11:19) swapinfo -h
Device 1K-blocks Used Avail Capacity
/dev/zvol/bigD/swap 1126400 94M 1G 9%
/dev/gpt/swap0.eli 3145728 93M 2.9G 3%
/dev/gpt/swap1.eli 3145728 94M 2.9G 3%
Total 7417856 282M 6.8G 4%
I guess that we are seeing some swap actually in use now because
buildkernel is running; i.e., > r241254.
The actual KVA requirements (RAM is only allocated when the swap space
is actually used) is about 5MB KVA per 1GB swap. The default swzone
for i386 was 32MiB - which is sufficient for ~7GB swap (the 1852656
pages reported above) and was increased to 34.5MB for i386 in r239730
to support ~8GB swap (this is also in r240097). (It's all approximate
because of the way swap space is allocated using struct swblock).
See the thread starting
http://lists.freebsd.org/pipermail/freebsd-current/2012-August/035839.html
for more details.
Interesting. Yet from my first reading it seems like the limit was
actually a warning and the warning still exists in amd64.
I wonder if there is to run other than swapinfo, to actually see if
swapinfo is correct in reporting that the swap in use is actually
identical in size from each disk of the mirror and also 94M precisely
from zfs swap?
Thank you,
Darrel
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