[issue32561] Add API to io objects for non-blocking reads/writes

[Nathaniel Smith]

Nathaniel Smith <njs at pobox.com> added the comment:

The idea here is *not* to avoid using a thread pool in general. When the data is on disk, using a thread pool is (a) unavoidable, because of how operating system kernels are written, and (b) basically fine anyway, because the overhead added by threads is swamped by the cost of disk access. So for the foreseeable future, we're always going to be using a thread pool for actual disk access.

But, if the data *is already in memory*, so the read can succeed without hitting the disk, then using a thread pool is *not* fine. Fetching data out of memory is super super cheap, so if that's all we're doing then using a thread pool adds massive overhead, in relative terms. We'd like to skip using the thread pool *specifically in this case*.

So the idea would be: first, attempt a "buffer-only" read. If it succeeds, then great we're done and it was really cheap. Otherwise, if it fails, then we know we're in the data-on-disk case, so we dispatch the operation to the thread pool.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue32561>
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