lowlevel-benchmarks

benchmarking ways of doing lowlevel work in dotnet.

TLDR;

1. Don't do cheap work in parallel. For example, in some of these benchmarks I get a 2x speedup for 16x the cpu cost.
2. For lookups, Span is fastest. Prefer Array/List for lookups in hotpaths over Dictionary/ConcurrentDictionary.
3. Avoid add/remove from ConcurrentDictionary in hotpaths due to allocations.
4. Interlocked is expensive to use in hotpaths. do per-thread sums or write out to a seperate results span for processing back on the main thread. Using a ForRange() parallel work function is best, for example: /Helpers/Extras/ForRange.cs
   • As per Kozi on the C# Discord:

     The more important lesson here is "don't write to the same memory region from multiple threads if possible". Writes within the same cache line will slow access on other threads. And THAT'S why doing it per-thread is better. And only summing at the end. You minimise the writes to a shared cache line. https://www.youtube.com/watch?v=WDIkqP4JbkE&t=247s

5. Linq and PLinq are not that bad. Not super great, but not that bad.
6. MemoryOwner<T> is your friend.

The Benchmarks

these are the benchmarks, contained in subfolders of /Benchmarks/. Look at each sub folder for a ReadMe.md with individual findings:

• Collections_Threaded checks speed/correctness of doing collection read/writes from threads
• Parallel_Work checks doing work on Span<T> from threads
• Parallel_Lookup checks a real-world critical path scenario, random access lookup of 100,000 entities. Benchmark tests using different backing storage collections and Sequential vs Parallel.

How to use

1. open solution in visual studio 2022
2. run solution
3. pick a benchmark
4. wait a long time for benchmarks to run

Folder Structure

• Program.cs - entrypoint
• Benchmarks/*/*.cs - benchmark tests
• Helpers - helpers for the benchmarking, such as:
  • DumbWork.cs - helper containing input data and output verification logic
  • Data.cs - helper containing structure of test data worked on in benchmarks
  • zz_Extensions.cs - extension method for Span<T> and Array to make parallel easier.