Hi Antoine, Thanks for posting this, and sorry for the delayed reply!
I've known about a possibility to optimize asyncio Protocols for a while. I noticed that `Protocol.data_received()` requires making one extra copy of the received data when I was working on the initial version of uvloop. Back then my main priority was to make uvloop fully compatible with asyncio, so I wasn't really thinking about improving asyncio design. Let me explain the current flaw of `Protocol.data_received()` so that other people on the list can catch up with the discussion: 1. Currently, when a Transport is reading data, it uses `sock.recv()` call, which returns a `bytes` object, which is then pushed to `Protocol.data_received()`. Every time `sock.recv()` is called, a new bytes object is allocated. 2. Typically, protocols need to accumulate bytes objects they receive until they have enough buffered data to be parsed. Usually a `deque` is used for that, less optimized code just concatenates all bytes objects into one. 3. When enough data is gathered and a protocol message can be parsed out of it, usually there's a need to concatenate a few buffers from the `deque` or get a slice of the concatenated buffer. At this point, we've copied the received data two times. I propose to add another Protocol base class to asyncio: BufferedProtocol. It won't have the 'data_received()' method, instead it will have 'get_buffer()' and 'buffer_updated(nbytes)' methods: class asyncio.BufferedProtocol: def get_buffer(self) -> memoryview: pass def buffer_updated(self, nbytes: int): pass When the protocol's transport is ready to receive data, it will call `protocol.get_buffer()`. The latter must return an object that implements the buffer protocol. The transport will request a writable buffer over the returned object and receive data *into* that buffer. When the `sock.recv_into(buffer)` call is done, `protocol.buffer_updated(nbytes)` method will be called. The number of bytes received into the buffer will be passed as a first argument. I've implemented the proposed design in uvloop (branch 'get_buffer', [1]) and adjusted your benchmark [2] to use it. Here are benchmark results from my machine (macOS): vanilla asyncio: 120-135 Mb/s uvloop: 320-330 Mb/s uvloop/get_buffer: 600-650 Mb/s. The benchmark is quite unstable, but it's clear that Protocol.get_buffer() allows to implement framing way more efficiently. I'm also working on porting asyncpg library to use get_buffer(), as it has a fairly good benchmark suite. So far I'm seeing 5-15% speed boost on all benchmarks. What's more important is that get_buffer() makes asyncpg buffer implementation simpler! I'm quite happy with these results and Ipropose to implement the get_buffer() API (or its equivalent) in Python 3.7. I've opened an issue [3] to discuss the implementation details. [1] https://github.com/MagicStack/uvloop/tree/get_buffer [2] https://gist.github.com/1st1/1c606e5b83ef0e9c41faf21564d75ad7 Thanks, Yury On Oct 18, 2017, 2:31 PM -0400, Antoine Pitrou <[email protected]>, wrote: > > Hi, > > I am currently looking into ways to optimize large data transfers for a > distributed computing framework > (https://github.com/dask/distributed/). We are using Tornado but the > question is more general, as it turns out that certain kinds of API are > an impediment to such optimizations. > > To put things short, there are a couple benchmarks discussed here: > https://github.com/tornadoweb/tornado/issues/2147#issuecomment-337187960 > > - for Tornado, this benchmark: > https://gist.github.com/pitrou/0f772867008d861c4aa2d2d7b846bbf0 > - for asyncio, this benchmark: > https://gist.github.com/pitrou/719e73c1df51e817d618186833a6e2cc > > Both implement a trivial form of framing using the "preferred" APIs of > each framework (IOStream for Tornado, Protocol for asyncio), and then > benchmark it over 100 MB frames using a simple echo client/server. > > The results (on Python 3.6) are interesting: > - vanilla asyncio achieves 350 MB/s > - vanilla Tornado achieves 400 MB/s > - asyncio + uvloop achieves 600 MB/s > - an optimized Tornado IOStream with a more sophisticated buffering > logic (https://github.com/tornadoweb/tornado/pull/2166) > achieves 700 MB/s > > The latter result is especially interesting. uvloop uses hand-crafted > Cython code + the C libuv library, still, a pure Python version of > Tornado does better thanks to an improved buffering logic in the > streaming layer. > > Even the Tornado result is not ideal. When profiling, we see that > 50% of the runtime is actual IO calls (socket.send and socket.recv), > but the rest is still overhead. Especially, buffering on the read side > still has costly memory copies (b''.join calls take 22% of the time!). > > For a framed layer, you shouldn't need so many copies. Once you've > read the frame length, you can allocate the frame upfront and read into > it. It is at odds, however, with the API exposed by asyncio's Protocol: > data_received() gives you a new bytes object as soon as data arrives. > It's already too late: a spurious memory copy will have to occur. > > Tornado's IOStream is less constrained, but it supports too many read > schemes (including several types of callbacks). So I crafted a limited > version of IOStream (*) that supports little functionality, but is able > to use socket.recv_into() when asked for a given number of bytes. When > benchmarked, this version achieves 950 MB/s. This is still without C > code! > > (*) see > https://github.com/tornadoweb/tornado/compare/master...pitrou:stream_readinto?expand=1 > > When profiling that limited version of IOStream, we see that 68% of the > runtime is actual IO calls (socket.send and socket.recv_into). > Still, 21% of the total runtime is spent allocating a 100 MB buffer for > each frame! That's 70% of the non-IO overhead! Whether or not there > are smart ways to reuse that writable buffer depends on how the > application intends to use data: does it throw it away before the next > read or not? It doesn't sound easily doable in the general case. > > > So I'm wondering which kind of APIs async libraries could expose to > make those use cases faster. I know curio and trio have socket objects > which would probably fit the bill. I don't know if there are > higher-level concepts that may be as adequate for achieving the highest > performance. > > Also, since asyncio is the de facto standard now, I wonder if asyncio > might grow such a new API. That may be troublesome: asyncio already > has Protocols and Streams, and people often complain about its > extensive API surface that's difficult for beginners :-) > > > Addendum: asyncio streams > ------------------------- > > I didn't think asyncio streams would be a good solution, but I still > wrote a benchmark variant for them out of curiosity, and it turns out I > was right. The results: > - vanilla asyncio streams achieve 300 MB/s > - asyncio + uvloop streams achieve 550 MB/s > > The benchmark script is at > https://gist.github.com/pitrou/202221ca9c9c74c0b48373ac89e15fd7 > > Regards > > Antoine. > > > _______________________________________________ > Async-sig mailing list > [email protected] > https://mail.python.org/mailman/listinfo/async-sig > Code of Conduct: https://www.python.org/psf/codeofconduct/
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