You're also missing the design limitations of the actual drives.
Assuming IDE/SATA, the disks do not support disconnected writes,
which is a significant performance bottleneck when you are writing to
the disk...SAS/SCSI drives have 128 concurrent writes (tagged command
queue depth).
I'm not sure what you mean by "also missing", since I have been spot-on
about disk writes potentially causing performance problems, and what
you're saying supports what I said before. This is something that I have
studied extensively.
You are misinformed about SATA drives. Many do support NCQ, which is the
equivalent to TCQ on SAS/SCSI. The OS also maintains its cache and uses
a scheduler to try to optimize writes, usually doing a decent job at
maintaining a good rate of IOPs. Regardless of the NCQ/TCQ capability,
the same performance problem would exist, given heavy enough disk access.
My comment about log writes listed them as an example of something that
can make a tick take longer than anticipated, along with plugins (and
the game itself). This is a valid example, but even if it were not
valid, the overall assertion stands.
> I have no idea what baseline performance is in the context of a
game server.
The baseline performance in that case would be no background disk
access.
mlock()? Memory backed filesystem that doesn't cause faults? different
drives? sockets? null?
I think there might be a misunderstanding here. My example was that disk
write delays due to logging during periods of heavy disk writes are one
factor that I have seen lead to a performance problem and at the same
time cause FPS dips. The baseline performance case for that particular
scenario is very simple and as I described. I was not suggesting that
there are no other reasons for FPS dips, or suggesting a baseline
performance description for all scenarios. This is also a very small
piece of what I said as a whole.
-John
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