Date: Wed, 27 Jun 2001 22:14:12 -0700 (PDT) From: Matt Dillon <dillon@earth.backplane.com> To: Doug Rabson <dfr@nlsystems.com> Cc: John Baldwin <jhb@FreeBSD.org>, Jonathan Lemon Alfred Perlstein <bright@sneakerz.org>, "Brian F. Feldman" <green@FreeBSD.org>, Mike Silbersack <silby@FreeBSD.org>, <cvs-committers@FreeBSD.org>, <cvs-all@FreeBSD.org>, <jlemon@FreeBSD.org>, <bmilekic@FreeBSD.org> Subject: Re: cvs commit: src/sys/netinet tcp_input.c tcp_output.c tcp_sub Message-ID: <200106280514.f5S5EC548320@earth.backplane.com> References: <Pine.BSF.4.33.0106272128380.96893-100000@herring.nlsystems.com>
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:Matt, I think you are forgetting that on BSD/OS, when a lightweight
:ithread hits a held mutex, it then converts to a fully scheduled
:heavyweight thread, at which point any kind of preemption you can think of
:can happen.
:...
:Doug Rabson Mail: dfr@nlsystems.com
Yes, but that only applies to the interrupt thread, not the original
thread that was interrupted, and it doesn't happen at a random point...
it happens at a well defined point. The issue here is that the way it's
implemented in FreeBSD, if the original non-interrupt thread is
preempted by an interrupt the original thread could resume on another
cpu, meaning that a thread cannot assume any sort of stable per-cpu state
at any time, which creates huge performance/efficiency problems.
BSDI didn't have this problem... a kernel thread could be preempted by
an interrupt, but ONLY by an interrupt, and the kernel thread would
always resume on the same cpu after the interrupt thread either completed
or had to be scheduled (with no other thread being able to get that cpu
in the mean time)... so in the BSDI implementation an interrupt works
pretty much the way it works in 4.x.
Thus in the BSDI scheme the cpu a thread is running on is 'stable'...
can't be changed out from under the thread at random points. This
allows threads to make significant optimizations in regards to accessing
per-cpu data structures.
-Matt
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