URLSession performance for reading a byte stream

Fri, 03 May 2024 23:52:00 +0000

What’s the best way to read a stream of bytes with URLSession? That’s the simple question I set out to answer. I wrote some benchmarks. They read a 128 MiB file and perform a contrived aggregation of its content bytes (a joking “hash” of them, merely to ensure the actual reads aren’t optimised out).

⚠️ In a nutshell, the results here demonstrate the best-case performance for each of the methods evaluated. These benchmarks are very simple, which makes them relatively easy for the Swift compiler to optimise well. In less trivial, real-world code, the optimiser might not do so great. So these benchmarks and their results are merely one collective data point in the bigger picture of just how the heck to read files efficiently.

There’s two key decisions you must make: which specific URLSession API will you use, and how will you access the bytes themselves.

Measurements

Each benchmark was run a hundred times or for 30 seconds (whichever limit was hit first). I’m highlighting here just the medians (in general there wasn’t much variation anyway), but you can dig into the other percentiles & metrics via the disclosure triangles, if you like.

I’m pretty sure the reads were all served out of the kernel’s in-memory file system cache, judging by the lack of SSD read I/O reported by iStat Menus. But I didn’t go out of my way to verify this.

⚠️ “Peak RAM” is as reported by the Benchmark package, based on (if I understand it correctly) periodic sampling of the process RSS. As such it’s not necessarily completely accurate, due to the potential to miss brief peaks.

M2 MacBook Air

Method	Wall time (ms)	CPU time (ms)	Throughput (MiB/s)	Peak RAM (MB)
`bytes(from:)` and for loop	79	138	1,620	91
`bytes(from:)` and `reduce`	79	138	1,620	84
`data(from:)` and for loop	605	641	212	265
`data(from:)` and for loop inside `withUnsafeBytes`	60	95	2,133	338
`data(from:)` and `forEach`	765	800	167	262
`data(from:)` and `reduce`	750	784	171	290
`dataTask(with:)` and an incremental delegate with for loop	560	617	229	53
`dataTask(with:)` and an incremental delegate with for loop inside `withUnsafeBytes`	36	75	3,556	26
`dataTask(with:)` and an incremental delegate with `forEach`	719	775	178	51
`dataTask(with:)` and an incremental delegate with `reduce`	709	765	167	45
`dataTask(with:completionHandler:)` and for loop	590	630	217	442
`dataTask(with:completionHandler:)` and for loop inside `withUnsafeBytes`	57	98	2,246	504
`dataTask(with:completionHandler:)` and `forEach`	742	783	173	470
`dataTask(with:completionHandler:)` and `reduce`	742	786	173	485

Full results (raw text)

NSData – Wade Tregaskis

URLSession performance for reading a byte stream

Measurements

M2 MacBook Air