RPI3 swap experiments

[bob prohaska]

On Wed, Jul 18, 2018 at 07:42:13AM -0700, Mark Millard wrote:
> 
> 
> On 2018-Jul-17, at 11:06 PM, bob prohaska <fbsd at www.zefox.net> wrote:
> 
> > It appears that some progress has been made in getting swap working reasonably
> > on the RPI3. A -j4 buildworld attempt running r336356 to compile 336431 failed
> > with "out of swap" but the worst read and write delays were less than 5 seconds,
> > a marked improvement over previous examples.
> 
> Attributing the time variations that have been observed mostly to FreeBSD and not
> mostly to the device at issue seems to have little or no evidence to support it.
> 
Possibly a fair objection. This test is with a USB3.0 flash drive. I'll repeat soon
as possible with a USB3.1 device, which in the past reported much greater (15 second)
delays.

> > In this case swap was split, 2 each 1GB  partitions on USB flash plus 1 GB on the 
> > microSD card. Previous attempts using 3 each 1 GB partitions on USB flash have been
> > repeatedly successful, while a single attempt using 3 each 1GB partitions on microSD
> > failed.
> 
> The more swap partitions (or space?) not on a /dev/mmcsd0s* the less of the
> activity that /dev/mmcsd0 handles and likely the more time it tends to have
> between explicit operations to do internal housekeeping before the next
> explicit operation.
> 
> So the better approximation to not using /dev/mmcsd0 at all might not be
> all that much of a surprise at having less of a problem on the device
> (or a problem less often).
> 
In earlier tests (same card type) putting _all_ swap on microSD (along with /tmp) 
avoided OOMA kills. Dependable mischief seems to come when swap is on both microSD 
and USB.

It's understood that USB and Ethernet share I/O hardware, but I thought microSD 
was at least somewhat independent. Is this wrong?

Thanks for reading,

bob prohaska