RPi2 i/o blocking and SD card performance

[Tim Kientzle]

> On Jun 9, 2016, at 2:37 AM, Gergely Imreh <imrehg at gmail.com> wrote:
> 
> Hi,
> 
> I've been testing FreeBSD 11.0-CURRENT on a RaspberryPi2. I'm relatively
> new to FreeBSD, and wondering if there's any advice for improving the
> performance a bit.
> 
> First, it looks like there's a lot of i/o blocking behaviour going on. For
> example running MediaWiki on the board, if I compile any ports, the site
> itself is pretty much unusable (the PHP scripts time out even with 180s
> timeouts). The strangest thing is that the CPU usage is not at 100% all the
> way, can be that all 4 cores are ~99% idle, and still everything goes very
> slow. Once the ports compilation or any other i/o-related task is finished,
> it's snappy again.
> 
> Any idea why it could be to have such big latency/lag even though the CPU
> is idle? Is there anything I could test?

Watch vmstat output for a while during these periods.  In the past, I’ve
noticed the CPU alternating between being completely busy and completely
idle.  When I last asked about this, I recall someone pointing to the disk buffer
management as the culprit — apparently, with slow devices such as SD cards
it tends to accumulate pending writes, then everything blocks while the disk
cache drains.

But I may have misunderstood...

Tim