On Tue, Jan 12, 2010 at 1:11 PM, Kurt Stephens <ks / kurtstephens.com> wrote:

> =A0If that's the case, then no one would use multi-core processors at all=
.
> =A0How the threads are mapped to a processor (i.e. another core or not) i=
s an
> OS issue.

Maybe you miss my point?  See below.

> =A0The issue here is that the majority of the work can be done in a separ=
ate
> thread which allows the mutator (in this case Ruby) to continue
> =A0while the GC's thread is doing its job.

If I have one core, under that particular GC scheme, my stuff runs 10% slow=
er.

If I have two cores, my stuff runs X% faster, because a bunch of the
GC work is offloaded to that second core.

So, if I have my code already running on two cores, there aren't any
more idle cores available to actually offload GC work to.  So my code
on both cores is now running 10% slower.

i.e. overall, we're doing more work to get a certain execution time or
throughput on our code using the Boehm conservative GC mentioned, but
that GC can spread the work out among more cores, so if the machine
isn't already running at max CPU capacity, that one job can appear to
run faster, but that breaks down if all of the cores are being
utilized for work.  For example, let's say that I have a 4 core
machine. I'm running some code on it that, when pushed to its limits,
utilizes all four cores at 100%.

If I'm running my code under the Boehm conservative GC, then so long
as only two or three cores are being pushed to their limits, or so
long as none of the four cores are being pushed too bar, there is
excess CPU capacity to handle the GC tasks, thus making my code run
with greater throughput.  However, as soon as all of the cores get
busy enough, the situation on the multicore machine is no different
than the situation would be on a single core machine -- my code has
execution times that are 10% longer.  So, when looking at the machine
as a whole, it's overall processing capacity, at least for CPU bound
tasks, gets reduced versus a GC that doesn't exact this tax.  It is
only a benefit when the cores are not being used to capacity.


Kirk Haines