Is maximum swap usage tunable?

[bob prohaska]

On Sat, Mar 03, 2018 at 08:26:05AM -0800, bob prohaska wrote:
> 
> Is there some sort of experiment which can distinguish hardware delays
> from software delays? For example, would logging gstat output shed any
> light? 
> 
For lack of any better ideas, I tried running
make -j2 -DNO_CLEAN buildworld > buildworld.log && make -j2 -DNO_CLEAN KERNCONF=
ZEFOX buildkernel > buildkernel.log

while also running
gstat -a -B -I 10s > j2_gstat.log & in another ssh session

In due course the console reported

FreeBSD/arm64 (www.zefox.org) (ttyu0)

login: Mar  4 09:28:30 www kernel: pid 9310 (c++), uid 0, was killed: out of swap space

as expected.

However, a grep of j2_gstat  revealed a maximum write delay of 30ms/w 
for swap on microSD. 

Swap on USB flash is slower, but still generally under 100 ms. 
Only a handful of widely spaced delays exceeded 200 ms/w. 
The worst-case events were
dT: 10.002s  w: 10.000s
 L(q)  ops/s    r/s   kBps   ms/r    w/s   kBps   ms/w   %busy Name

    0      6      0      0    0.0      6    113   14.6    3.3  da0b
    0      4      0      0    0.0      4     48   29.0    3.1  da0b
    0      9      5     79    3.0      5     47    7.9    2.6  da0b
    4      8      0      0    0.0      8     99   67.5   32.5  da0b
    0      1      0     13    5.6      1     28   5674   88.3  da0b
    0      0      0      0    0.0      0     38   18.6    0.3  da0b
    0      1      1      9    2.8      0      0    0.0    0.2  da0b
    0      1      1     26    5.1      0      0    0.0    0.4  da0b
    0      0      0      3    2.6      0      0    0.0    0.1  da0b
    0      1      1      9  161.8      0      0    0.0   14.3  da0b

No "indefinite delay" warnings were presented on the console.
uname -a reports r329893, sources are at 330383.

I hope this is useful information,  

bob prohaska