Firefox slowdown during big IO ops

[Peter]

It is customary to lower the scheduling priority of the large IO processes and
in general of all heavy background tasks. The slowdown affects all programs but
especially those which rely on polling or selecting on IO streams with timeouts.
It has to do with the IO scheduling policy and with the type of preemptive-ness
and IO policy the kernel was compiled with. There are kernel parameters to tune
that. Server testing under load usually includes generating this type of load
and the measures to improve the response are discussed in such contexts. On
real-time kernels (CNC Linux etc) the problem does not exist, and for good
reason, by design. There is no single answer to the problem.

The problem also occurs when the system is swapping a lot, regardless whether
the swapped pages are related to FF or to the server process which is involved.

-- Peter

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