[Python-ideas] The async API of the future

Guido van Rossum guido at python.org
Sat Nov 3 00:59:49 CET 2012

Working code or it didn't happen. (And it should scale too.)

--Guido van Rossum (sent from Android phone)
On Nov 2, 2012 2:58 PM, "Sturla Molden" <sturla at molden.no> wrote:

> Den 19. okt. 2012 kl. 18:05 skrev Guido van Rossum <guido at python.org>:
> > An issue in the design of the I/O loop is the strain between a
> > ready-based and completion-based design. The typical Unix design
> > (whether based on select or any of the poll variants) is usually
> > ready-based; but on Windows, the only way to get high performance is
> > to base it on IOCP, which is completion-based (i.e. you start a
> > specific async operation, like writing N bytes, and the I/O loop tells
> > you when it is done). I would like people to be able to write fast
> > event handling programs on Windows too, and ideally the only change
> > would be the implementation of the I/O loop. But I don't know how
> > tenable that is given the dramatically different style used by IOCP
> > and the need to use native Windows API for all async I/O -- it sounds
> > like we could only do this if the library providing the I/O loop
> > implementation also wrapped all I/O operations, andthat may be a bit
> > much.
> Not really, no.
> IOCP might be the easiest way to get high performance on Windows, but
> certainly not the only.
> IOCP is a simple user-space wrapper for a thread-pool and overlapped (i.e.
> asynchronous) i/o. There is nothing IOCP can do that cannot be done with a
> pool of threads and non-blocking read or write operations.
> Windows certainly has a function to select among multiple wait objects,
> called WaitForMultipleObjects. If open files are associated with event
> objects signalling "ready-to-read" or "ready-to-write", that is the basic
> machinery of an Unix select() function.
> Then the problem is polling for "ready-to-read" and "ready-to-write". The
> annoying part is that different types of files (disk files, sockets, pipes,
> named pipes, hardware devices) must be polled with different Windows API
> calls – but there are non-blocking calls to poll them all. For this reason,
> Cygwin's select function spawn one thread to poll each type of file.
> Threads are very cheap on Windows, and polling loops can use Sleep(0) to
> relese the remainder of their time-slice, so this kind of polling is not
> very expensive. However, if we use a thread-pool for the polling, instead
> of spawing new threads on each call to select, we would be doing more or
> less the same as Windows built-in IOCPs, except we are signalling "ready"
> instead of "finished".
> Thus, I think it is possible to get high performance without IOCP. But
> Microsoft has only implemented a select call for sockets. My suggestion
> would be to forget about IOCP and implement select for more than just
> sockets on Windows. The reason for this is that select and IOCP are
> signalling on different side of the I/O operation (ready vs. completed). So
> programs based on select ans IOCP tend to have opposite logics with respect
> to scheduling I/O. And as the general trend today is to develop for Unix
> and then port to Windows (as most programmers find the Windows API
> annoying), I think it would be better to port select (and perhaps poll and
> epoll) to Windows than provide IOCP to Python.
> Sturla
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