cvs commit: src/sys/ddb db_ps.c src/sys/kern init_main.c kern_exit.c kern_fork.c kern_resource.c kern_sig.c sys_generic.c src/sys/platform/pc32/i386 pmap.c src/sys/platform/vkernel/platform pmap.c src/sys/sys proc.h tree.h src/sys/vm vm_vmspace.c
dillon at apollo.backplane.com
Wed Aug 15 10:12:21 PDT 2007
:The problem is that sel*() is anyways O(nproc), because it only stores the PID and not the proc pointer. So that is definitely an area for optimization.
:Serve - BSD +++ RENT this banner advert +++ ASCII Ribbon /"\
pid lookups are nearly O(1). They run through a fairly large hash
table. That isn't the problem with select. If you are thinking of
storing process and lwp pointers in selrecord(), its just not possible.
The information in selrecord() can become stale very quickly, which is
why it has to validate the pids/lwpids stored there to determine whether
those processes and threads actually still exist. Fortunately for
selrecord() all those lookups can be shortcutted when the calling thread
matches the one already recorded.
The main problem with select is when it has to set the collision bit,
which occurs basically every time more then one thread selects on the
same descriptor. At that point selrecord has to set SI_COLL and
selwakeup will wake up all the threads blocked on the select instead
of just one.
So for example, if you are running apache configured to select on
accept (apache has options to get around it now but lets just say...),
and you had 16 apache threads able to accept connections, then all
16 threads would be woken up and return from the select() call
for each incoming connection.
There is also the problem of select() having to poll every single
descriptor passed into it which can result in horrendous overhead,
but there isn't much we can do about that.
Nor is there a whole lot we can do about select() either. If we
implement code to wake up only one select we risk racing so many
other things that there's a good chance the program may miss the
accept (or other I/O) event entirely.
<dillon at backplane.com>
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