Performance expectations

[AdamSimpson]

cacache version: v11.3.0
rustc: 1.69.0

I'd like to use cacache in a performance sensitive area of my application, but I have found there to be more overhead than hoped. For instance on my M2 MacBook Air(2,800 MBps read SSD) it takes ~90ms to read a 30MB file from cache(sync or async), while reading the file directly from disk takes ~15ms. I see similar performance within my application as well as within microbenchmarks, so I don't believe this is simply bad benchmarking.

I'm not familiar with the caching strategy, so maybe this is expected? Possibly a system(MacOS) specific issue? Bad benchmarking?

[zkat]

Cacache will be strictly slower than direct file access because it builds on top of it and involves a bit of overhead. There "official" benchmarks I use to compare various scenarios tend to show smaller gaps than what you're reporting, though.

The first thing that comes to mind is that there will definitely be a big gap between sync and async access.

[AdamSimpson]

I added a few timers to read() and it looks like the integrety check is very time consuming. I don't have any feeling for if this is a reasonable amount of time for the check, but it was unexpected.

FWIW I also tested on a linux box and see similar perf.

content_path time: 1.458µs

read time: 3.023791ms

check time: 86.757209ms

[zkat]

I guess that's not entirely unexpected but it definitely feels like more than it should be. Then again, sha256 can be expensive?

[AdamSimpson]

I ran some benchmarks with the sha2 crate and on my system it looks to be the cost of business. I also tested xxhash, for some use cases that might be of interest :)

sha256_30mb_file        time:   [87.594 ms 87.714 ms 87.840 ms]                             
Found 5 outliers among 100 measurements (5.00%)
  5 (5.00%) high mild

xxhash_30mb_file        time:   [1.9958 ms 1.9993 ms 2.0033 ms]                              
Found 6 outliers among 100 measurements (6.00%)
  4 (4.00%) high mild
  2 (2.00%) high severe

[zkat]

It might be interesting to make xxhash or a similar high-perf non-crypto algo available behind a feature flag. All my use cases for cacache necessitate cryptographic algos, though, so it's unlikely this is something I would do myself. Happy to take a PR, though!

[zkat]

After some thinking, I've decided to add xxhash support to cacache, as I've found a specific use case where I don't need the cryptographic guarantees of sha2. Assigning this to myself!

[zkat]

@AdamSimpson I've added xxhash support and released it as part of v11.6.0. Wanna give it a whirl and tell me what you think? :)

[AdamSimpson]

So sorry for missing this, but this is perfect. Thank you so much!