[Haskell-cafe] STM friendly TreeMap (or similar with range scan api) ? WAS: Best ways to achieve throughput, for large M:N ratio of STM threads, with hot TVar updates?

Peter Robinson pwr at lowerbound.io
Thu Jul 30 11:19:45 UTC 2020

Hi Compl,

> >+ This package provides a proof-of-concept implementation of a skip list
>> in STM
>> This has to mean something but I can't figure out yet.
>> Dear Peter Robinson, I hope you can see this message and get in the loop
>> of discussion.
 The reason for adding this sentence was that tskiplist hasn't been
optimized for production use. Later on, I wrote an implementation of a
concurrent skip list with atomic operations that performs significantly
better, but it's operations work in the IO monad.

I'm surprised to hear that you're getting poor performance even when using
the stm-container package, which I believe was meant to be used in
production. A while ago, I ran some benchmarks comparing concurrent
dictionary data structures (such as stm-container) under various workloads.
While STMContainers.Map wasn't as fast as the concurrent-hashtable package,
the results indicate that the performance doesn't degrade too much under
larger workloads.

You can find these benchmark results here (10^6 randomly generated
insertion/deletion/lookup requests distributed among 32 threads):
And some explanations about the benchmarks are here:

One issue that I came across when implementing the tskiplist package was
this: If a thread wants to insert some item into the skip list, it needs to
search for the entry point by performing readTVar operations starting at
the list head. So, on average, a thread will read O(log n) TVars (assuming
a skip list of n items) and, if any of these O(log n) TVars are modified by
a simultaneously running thread, the STM runtime will observe a (false)
conflict and rerun the transaction. It's not clear to me how to resolve
this issue without access to something like unreadTVar (see [1]).


[1] UnreadTVar: Extending Haskell Software Transactional Memory for
Performance (2007)  by Nehir Sonmez , Cristian Perfumo , Srdjan Stipic ,
Adrian Cristal , Osman S. Unsal , Mateo Valero.
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