That code will not switch to another thread if the condition variable either does not actually block the thread on the call (which is allowed by the standard to give implementations some flexibility for making it work correctly � read the standard reasoning for more information), or the thread is woken up without a predicate change (also allowed by the standard for similar reasons). Both of those cases are called �spurious wake-ups�.
">

(original) (raw)

There is no need for the "forced" switch to be�guaranteed. The motivation for the wait is to increase throughput and decrease latency when OS schedules next thread to run on same core as yielding thread but yielding thread is about to continue running into CPU intensive library code. An occasional spurious wakeup resulting in missed switch will not affect that.�Note similar logic in new GIL.

I will add the loop to keep the code clean.�

Thanks,
Nir



On Mon, Apr 12, 2010 at 10:49 PM, Peter Portante <peter.a.portante@gmail.com> wrote:
That code will not switch to another thread if the condition variable either does not actually block the thread on the call (which is allowed by the standard to give implementations some flexibility for making it work correctly � read the standard reasoning for more information), or the thread is woken up without a predicate change (also allowed by the standard for similar reasons). Both of those cases are called �spurious wake-ups�.

You may not be able to readily get your implementation to behavior that way, but in the wild, we need to account for this behavior because Cpython will be run on systems where it will happen. :)

\-peter



On 4/12/10 3:36 PM, "Nir Aides" <nir@winpdb.org> wrote:

Please describe clearly a step by step scenario in which that code will fail.


On Mon, Apr 12, 2010 at 10:25 PM, Peter Portante <peter.a.portante@gmail.com> wrote:
Yes, but unless you loop until the predicate is False, no forced switch is guaranteed to occur. You might as well remove the code. If you want to keep the code as is, call me when you need a life guard to help debug mystifying behaviors. ;) -peter



On 4/12/10 3:17 PM, "Nir Aides" <nir@winpdb.org <http://nir@winpdb.org> > wrote:

The loop-less wait is similar to the one in new GIL. It is used to force a switch to next thread in particular scenario and the motivation is explained in comment to another if clause a few lines up. Those two if clauses can be joined though.


On Mon, Apr 12, 2010 at 3:37 PM, Peter Portante <peter.a.portante@gmail.com <http://peter.a.portante@gmail.com> > wrote:
Hmm, so I see in
bfs\_yield():

+ ���if (tstate != NULL && bfs\_thread\_switch == tstate) {
\+ �������COND\_RESET(tstate->bfs\_cond);
\+ �������COND\_WAIT(tstate->bfs\_cond, bfs\_mutex);
\+ ���}

So, to me, the above code says, �Wait for the condition that tstate is either NULL, or bfs\_thread\_switch does not equal tstate�. So the predicate is: �(tstate != NULL && bfs\_thread\_switch == tstate)�.

If the predicate is True before you call COND\_WAIT() and True after you call COND\_WAIT(), either you don�t need to call COND\_WAIT() at all, or you need to loop until the predicate is False. There is no guarantee that a condition wait actually did anything at all. Yes, there will be spurious wake ups, but you can�t tell if the thread actually blocked and then woke up, or never blocked at all. If it never actually blocks, then that code path is not helpful.

On Windows, before this loop in bfs\_schedule():

+ �������COND\_RESET(tstate->bfs\_cond);
\+ �������while (bfs\_thread != tstate) {
\+ �����������\_bfs\_timed\_wait(tstate, timestamp);
\+ �����������timestamp = get\_timestamp();
\+ �������}

You might want to avoid the call to reset the condition variable if the predicate is already False.

\-peter



On 4/12/10 8:12 AM, "Nir Aides" <nir@winpdb.org <http://nir@winpdb.org> �<http://nir@winpdb.org> > wrote:

Hi Peter,

There is no need for a loop in bfs\_yield().�


On Mon, Apr 12, 2010 at 4:26 AM, Peter Portante <peter.a.portante@gmail.com <http://peter.a.portante@gmail.com> �<http://peter.a.portante@gmail.com> > wrote:
Nir,

Per the POSIX standard, both pthread\_cond\_wait() and pthread\_cond\_timedwait() need to be performed in a loop. �See the fourth paragraph of the description from:

http://www.opengroup.org/onlinepubs/000095399/functions/pthread\_cond\_timedwait.html <http://www.opengroup.org/onlinepubs/000095399/functions/pthread\_cond\_timedwait.html>



For the Windows side, I think you have a similar problem. Condition variables are signaling mechanisms, and so they have a separate boolean predicate associated with them. If you release the mutex that protects the predicate, then after you reacquire the mutex, you have to reevaluate the predicate to ensure that the condition has actually been met.


You might want to look at the following for a discussion (not sure how good it is, as I just google�d it quickly) of how to implement POSIX semantics on Windows:

http://www.cs.wustl.edu/\~schmidt/win32-cv-1.html <http://www.cs.wustl.edu/\~schmidt/win32-cv-1.html>



Before you can evaluate the effectiveness of any of the proposed scheduling schemes, the fundamental uses of mutexes and condition variables, and their implementations, must be sound.



-peter







On 4/11/10 6:50 PM, "Nir Aides" <



Hello all,

I would like to kick this discussion back to life with a simplified implementation of the BFS scheduler, designed by the Linux kernel hacker Con Kolivas: http://ck.kolivas.org/patches/bfs/sched-BFS.txt <http://ck.kolivas.org/patches/bfs/sched-BFS.txt>



I submitted bfs.patch at� http://bugs.python.org/issue7946 <http://bugs.python.org/issue7946> . It is work in progress but is ready for some opinion.



On my machine BFS gives comparable performance to gilinter, and seems to schedule threads more fairly, predictably, and with lower rate of context switching.�Its basic design is very simple but nevertheless it was designed by an expert in this field, two�characteristics�which combine to make it attractive to this case.

The problem addressed by the GIL has always been \*scheduling\* threads to the interpreter, not just controlling access to it, and therefore the GIL, a lock implemented as a simple semaphore was the wrong solution.

The patches by Antoine and David essentially evolve the GIL into a scheduler, however both cause thread starvation or high rate of context switching in some scenarios:

With Floren't write test ( http://bugs.python.org/issue7946#msg101120 <http://bugs.python.org/issue7946#msg101120> ):

2 bg threads, 2 cores set to performance, karmic, PyCon patch, context switching shoots up to 200K/s.
2 bg threads, 1 core, set to on-demand, karmic, idle machine, gilinter patch starves one of the bg threads.
4 bg threads, 4x1 core xeon, centos 5.3, gilinter patch, all bg threads starved, context switching shoots up to 250K/s.

With UDP test ( http://bugs.python.org/file16316/udp-iotest.py <http://bugs.python.org/file16316/udp-iotest.py> ), add zlib.compress(b'GIL') to the workload:

both gilinter and PyCon patches starve the IO thread.

The BFS patch currently involves more overhead by reading the time stamp on each yield and schedule operations. In addition it still remains to address some issues related to timestamps such as getting different time stamp readings on different cores on some (older) multi-core systems.

Any thoughts?

Nir



On Sun, Mar 14, 2010 at 12:46 AM, Antoine Pitrou <



Hello,



As some of you may know, Dave Beazley recently exhibited a situation

where the new GIL shows quite a poor behaviour (the old GIL isn't very

good either, but still a little better). This issue is followed in

http://bugs.python.org/issue7946 <http://bugs.python.org/issue7946>



This situation is when an IO-bound thread wants to process a lot of

incoming packets, while one (or several) CPU-bound thread is also

running. Each time the IO-bound thread releases the GIL, the CPU-bound

thread gets it and keeps holding it for at least 5 milliseconds

(default setting), which limits the number of individual packets which

can be recv()'ed and processed per second.



I have proposed two mechanisms, based on the same idea: IO-bound

threads should be able to steal the GIL very quickly, rather than

having to wait for the whole "thread switching interval" (again, 5 ms

by default). They differ in how they detect an "IO-bound threads":



- the first mechanism is actually the same mechanism which was

��embodied in the original new GIL patch before being removed. In this

��approach, IO methods (such as socket.read() in socketmodule.c)

��releasing the GIL must use a separate C macro when trying to get the

��GIL back again.



- the second mechanism dynamically computes the "interactiveness" of a

��thread and allows interactive threads to steal the GIL quickly. In

��this approach, IO methods don't have to be modified at all.



Both approaches show similar benchmark results (for the benchmarks

that I know of) and basically fix the issue put forward by Dave Beazley.



Any thoughts?



Regards



Antoine.





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