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Comment by olix0r | Hacker News Reader

> Just as dashmap is a replacement for std::sync::RwLock<HashMap>, whirlwind aims to be a replacement for tokio::sync::RwLock<HashMap>.

I'm curious about the practical benefits of handling a HashMap with an async interface. My long-standing understanding is that `tokio::sync::RwLock<HashMap>` is only useful when you'd want to hold the lock guard across another `await` operation; but when the lock guard is not held across an await, it is always preferable to use the synchronous version.

This would lead me to assume that same applies for dashmap--it should be sufficient for async use cases and doesn't need an async API unless we expect to be blocked, but the benchmarks indicate that whirlwind outperforms dashmap in various situations. Do you have a sense of where this blocking occurs in dashmap?

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The blocking mainly occurs due to contention - imo most of the performance gain comes from being able to poll the lock instead of blocking until it's available when acquiring locks on shards.

In all honesty I was quite surprised by the benchmarks as well though, I wouldn't expect that much performance gain, but in high-contention scenarios it definitely makes sense.

Benchmarks will always look good when using a spin-lock like you seem to be using here https://github.com/fortress-build/whirlwind/blob/0e4ae5a2aba...

I believe that doesn't indicate it's spinlocking; the `poll` API is specified to not block.

> In software engineering, a spinlock is a lock that causes a thread trying to acquire it to simply wait in a loop ("spin") while repeatedly checking whether the lock is available

Immediately waking itself, the task is scheduled and will be polled again shortly. This creates the loop in which is checks if the lock is available. This has no effective difference compared to using hint::spin_loop() but in async.

A more traditional lock will use a queue in which the task will not be polled until it's likely available, rather than blindly trying on repeat.

The code linked does the following:

- attempt to acquire an RWLock without blocking

- if it does, wake the task waiting for the lock

- if it doesn't, return "not ready yet"

The RWLock is straight from the standard lib; there's no loop and no spinning involved. Every single Future you look at will look like this, by specification they cannot block, only return "ready" (and wake the task), or return "not ready".

By waking itself, the task goes back to the scheduler queue. It then returns pending, going back to the scheduler where it picks a new task. It will eventually pick the scheduled task again and poll the rwlock. This is a loop, even if it's a long-winded one, and is by all accounts spinning.

It really is not much different to a thread blocking on a loop, where the OS thread scheduler will pre-empt the thread, rescheduling it for later. In this case it's an async task instead of a thread, and cooperative instead of preemptive, but it's still unfair and not very effective use of scheduler resources

Oh my mistake. I wonder what the benchmarks would look like without the wake loop

> Every single Future you look at will look like this,

That's not true. A Future is supposed to schedule itself to be woken up again when it's ready. This Future schedules it to be woken immediately. Most runtimes, like Tokio, will put a Future that acts like this at the end of the run queue, so in practice it's not as egregious. However, it's unquestionably a spin lock, equivalent to back off with thread::yield.

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