Bytes before FLOPS: your algorithm is (mostly) fine, your data isn't
bitsdraumar.is33 points by bofersen a day ago
33 points by bofersen a day ago
To the list of profiling tools I would like to add KDAB Hotspot and KDE Heaptrack.
The former, hotspot, is a visualiser for perf data, and it deals ok with truly massive files that made perfetto and similar just big down. It also supports visualing off-CPU profiles ("why is my program slow but not CPU bound?").
The latter, heaptrack, is a tool with very similar UI to hotspot (I think the two tools share some code even) to profile malloc/free (or new/delete). Sometimes the performance issue is as simple as not reusing a buffer but reallocating it over and over inside a loop. And sometimes you wonder where all the memory is going.
Great article. Can confirm, writing performance focused C# is fun. It's great having the convenience of async, LINQ, and GC for writing non-hot path "control plane" code, then pulling out Vector<T>, Span<T>, and so on for the hot path.
One question, how portable are performance benefits from tweaks to memory alignment? Is this something where going beyond rough heuristics (sequential access = good, order of magnitude cache sizes, etc) requires knowing exactly what platform you're targeting?
Author here. First of all, thanks for the compliment! It’s tough to get myself to write these days, so any motivation is appreciated.
And yes, once all the usual tricks have been exhausted, the nest step is looking at the cache/cache line sizes of the exact CPU you’re targeting and dividing the workload into units that fit inside the (lowest level possible) cache, so it’s always hot. And if you’re into this stuff, then you’re probably aware of cache-oblivious algorithms[0] as well :)
Personally, I almost never had the need to go too far into platform-specific code (except SIMD, of course), doing all the stuff in the post is 99% of the way there.
And yeah, C# is criminally underrated, I might write a post comparing high-perf code in C++ and C# in the future.
[0]: https://en.wikipedia.org/wiki/Cache-oblivious_algorithm
> worst case scenario being the flat profile where program time is roughly evenly distributed
It sounds like the “worst case“ here is that the program is already optimized.
Author here, kinda sorta. I should've been a bit more specific than that. You can have a profile showing a function taking up 99% of the time, but when you dive into it, there's no clear bottleneck. But just because there's no bottleneck, that doesn't mean it's optimized; vice versa-a well-optimized program can have a bottleneck that's already been cycle-squeezed to hell and back.
What I wanted to say was that a spiky profile provides a clear path to optimizing a piece of code, whereas a flat profile usually means there are more fundamental issues (inefficient memory management, pointer chasing all over the place, convoluted object system, etc.).
It sounds like a flat profile essentially is a local optimum, compared to cases where there's a path "upwards" along a hill to some place more optimal that doesn't require completely changing your strategy.
That's actually a good observation, yeah. It's often the case that you dig deeper and deeper and find some incomprehensible spaghetti and just say "fuck it, I'll just do what I can here, should be enough".