Re: To thread or not to thread ?

"James Kanze" <>
18 Jan 2007 12:54:24 -0500
Carlos Moreno wrote:

JohnQ wrote:

It depends on the application. You generally will not tolerate
the loss of responsivity just because one or two clients are
performing extremely complex requests.

OK, you're thinking SQL server and I'm thinking OLTP server.

Somehow this subthread went soooo far off a tangent!! :-)

That seems to be the status of a number of threads at the

My initial point was simple: provided that you decide that you
want multiple threads (given the way you decide your software
design goes), then the rule concerning how many threads (or
rather, whether or not more threads than available CPU cores)
is simple:

If the task involved (in this case, database processing) involves
a non-negligible fraction of tasks that are IO-bound, such as
reading or writing the disk, sending or receiving through the
network, or any other tasks that block the CPU into an idle
state of doing nothing while waiting for some event to happen
or to be completed, *then*, having more active threads than
available CPUs is bound to improve performance, responsiveness,

And the point of my posting (to which JohnQ was responding) is
that there can be other reasons to use more threads, such as
responsiveness. I write large scale servers for a
living---on-line transaction systems, if you want a buzz word,
but I was writing them well before the acronym existed. Some of
the requests the server receives may require extensive CPU time
(up to five minutes); one of the motivations for threading is
(or was---when the system was designed, we only had a single
processor system) to ensure responsiveness for the simple
requests: if a simple request occurs during the processing of a
long request, it will still get treated, without waiting for the
long request to terminate.

This does increase the total CPU load, because of the added time
for context swapping, etc. But that's not the issue here; most
of the time, the server isn't doing anything anyway. The
important point is that a user who makes a simple request gets a
fast answer, regardless of what other users might be doing at
the time.

      [BTW: I've put follow-ups to comp.programming.threads. This
    really has nothing to do with C++.]

James Kanze (GABI Software)
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