GPT vs MBR for swap devices

[bob prohaska]

On Mon, Jun 18, 2018 at 04:42:21PM -0700, Mark Millard wrote:
> 
> 
> On 2018-Jun-18, at 4:04 PM, bob prohaska <fbsd at www.zefox.net> wrote:
> 
> > On Sat, Jun 16, 2018 at 04:03:06PM -0700, Mark Millard wrote:
> >> 
> >> Since the "multiple swap partitions across multiple
> >> devices" context (my description) is what has problems,
> >> it would be interesting to see swapinfo information
> >> from around the time frame of the failures: how much is
> >> used vs. available on each swap partition? Is only one
> >> being (significantly) used? The small one (1 GiByte)?
> >> 
> > There are some preliminary observations at
> > 
> > http://www.zefox.net/~fbsd/rpi3/swaptests/newtests/1gbusbflash_1gbsdflash_swapinfo/1gbusbflash_1gbsdflash_swapinfo.log
> > 
> > If you search for 09:44: (the time of the OOM kills) it looks like
> > both swap partitions are equally used, but only 8% full.
> > 
> > At this point I'm wondering if the gstat interval (presently 10 seconds)
> > might well be shortened and the ten second sleep eliminated. On the runs
> > that succeed swap usage changes little in twenty seconds, but the failures
> > seem to to culminate rather briskly.
> 
> One thing I find interesting somewhat before the OOM activity is
> the 12355 ms/w and 12318 ms/w on da0 and da0d that goes along
> with having 46 or 33 L(q) and large %busy figures in the same
> lines --and 0 w/s on every line:
> 
> Mon Jun 18 09:42:05 PDT 2018
> Device          1K-blocks     Used    Avail Capacity
> /dev/da0b         1048576     3412  1045164     0%
> /dev/mmcsd0s3b    1048576     3508  1045068     0%
> Total             2097152     6920  2090232     0%
> dT: 10.043s  w: 10.000s
>  L(q)  ops/s    r/s   kBps   ms/r    w/s   kBps   ms/w    d/s   kBps   ms/d   %busy Name
>     0      0      0      0    0.0      0      9   10.8      0      0    0.0    0.1  mmcsd0
>    46      0      0      0    0.0      0     16  12355      0      0    0.0   85.9  da0
>     0      0      0      0    0.0      0      9   10.8      0      0    0.0    0.1  mmcsd0s3
>     0      0      0      0    0.0      0      9   10.8      0      0    0.0    0.1  mmcsd0s3a
>    33      0      0      0    0.0      0     22  12318      0      0    0.0  114.1  da0d
> Mon Jun 18 09:42:25 PDT 2018
> Device          1K-blocks     Used    Avail Capacity
> /dev/da0b         1048576     3412  1045164     0%
> /dev/mmcsd0s3b    1048576     3508  1045068     0%
> Total             2097152     6920  2090232     0%
> 
> 
> The kBps figures for the writes are not very big above.
> 

If it takes 12 seconds to write, I can understand the swapper getting impatient....
However, the delay is on /usr, not swap.

In the subsequent 1 GB USB flash-alone test case at
http://www.zefox.net/~fbsd/rpi3/swaptests/newtests/1gbusbflash_swapinfo/1gbusbflash_swapinfo.log
the worst-case seems to be at time 13:45:00

dT: 13.298s  w: 10.000s
 L(q)  ops/s    r/s   kBps   ms/r    w/s   kBps   ms/w    d/s   kBps   ms/d   %busy Name
    0      0      0      0    0.0      0      5    5.5      0      0    0.0    0.1  mmcsd0
    9     84      0      0    0.0     84   1237   59.6      0      0    0.0   94.1  da0
    0      0      0      0    0.0      0      5    5.5      0      0    0.0    0.1  mmcsd0s3
    0      0      0      0    0.0      0      5    5.6      0      0    0.0    0.1  mmcsd0s3a
    5     80      0      0    0.0     80   1235   47.2      0      0    0.0   94.1  da0b
    4      0      0      0    0.0      0      1   88.1      0      0    0.0    0.7  da0d
Mon Jun 18 13:45:00 PDT 2018
Device          1K-blocks     Used    Avail Capacity
/dev/da0b         1048576    22872  1025704     2%

1.2 MB/s writing to swap seems not too shabby, hardly reason to kill a process.

Thus far I'm baffled. Any suggestions?

Thanks for reading!

bob prohaska