[Python-ideas] Add faster locks to the threading module? (original) (raw)

Cameron Simpson cs at zip.com.au
Wed Jul 21 08:22:27 CEST 2010

On 21Jul2010 02:51, Sturla Molden <sturla at molden.no> wrote: | Thread synchronization with threading.Lock can be expensive. But | consider this: Why should the active thread need to aquire a mutex, | when it already holds one? That would be the GIL. || Instead of acqiring a lock (and possibly inducing thread swiching | etc.), it could just deny the other threads access to the GIL for a | while. The cost of that synchronization method would be completely | amortized, as check intervals happen anyway. [...] | The usecase for this "gillock" is about the same as for a spinlock | C. We want synchronization for a breif period of time, but don't | want the overhead of aquiring a mutex.

But a spinlock does acquire a mutex, unless I misremember.

It is just that instead of using a more expensive mutex mechanism that deschedules the caller until available it sits there banging its head against a very lightweight (not descheduling) mutex until it gets it; the efficiency is entirely reliant on all users of the spinlock doing very little inside the located period.

And therein lies one of my two misgivings about this idea: users of this must be very careful at all times to do Very Little inside the lock. Just like a spinlock. This seems very easy to misuse.

The other misgiving is one you've already mentioned in passing:

P.S. A gillock working like the ctypes code above is of course very dangerous. If a C extension releases the GIL while _Py_Ticker is astronomic, we have a very bad situation... But real code could try to safeguard against this. E.g. Py_BEGIN_ALLOW_THREADS and Py_END_ALLOW_THREADS could be defined to do nothing if _Py_Ticker is above some ridiculous threshold.

Suppose someone goes:

with threadsafe(): x = fp.read(1)

and the read blocks? It looks like trivial code but I think the I/O stuff will release the GIL over a read. Now you're not safe any more.

Cheers,

Cameron Simpson <cs at zip.com.au> DoD#743 http://www.cskk.ezoshosting.com/cs/

The ZZR-1100 is not the bike for me, but the day they invent "nerf" roads and ban radars I'll be the first in line......AMCN

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