TMap vs immutable Map in a Ref

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Kevin Esler

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Jun 10, 2013, 11:14:27 AM6/10/13
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I'm new to STM so go easy on the newbie. I have an Akka app in which various actors need to share a cache so STM looks like the thing to use.

The cache has the semantics of a Scala Map. I see I could use TMap, but it seems like I could also just store the cache as an immutable Map inside a Ref.

What are the pros and cons of these two alternatives?


Nathan Bronson

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Jun 10, 2013, 1:15:38 PM6/10/13
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Kevin,

Both approaches will have the right behavior, but TMap will give you more concurrency. The STM checks whether transactions can safely run in parallel at the granularity of a Ref. Ref[Map] has only a single ref, so if any thread writes to the map then all of the other transactions will have to wait to read. TMap a hash trie built of Ref-s, so transactions can simultaneously write different leaves without colliding.

 - Nathan


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Kevin Esler

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Jun 10, 2013, 3:34:15 PM6/10/13
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Thanks. That makes perfect sense.


On Monday, June 10, 2013 1:15:38 PM UTC-4, Nathan Bronson wrote:
Kevin,

Both approaches will have the right behavior, but TMap will give you more concurrency. The STM checks whether transactions can safely run in parallel at the granularity of a Ref. Ref[Map] has only a single ref, so if any thread writes to the map then all of the other transactions will have to wait to read. TMap a hash trie built of Ref-s, so transactions can simultaneously write different leaves without colliding.

 - Nathan
On Mon, Jun 10, 2013 at 8:14 AM, Kevin Esler <kevin...@gmail.com> wrote:
I'm new to STM so go easy on the newbie. I have an Akka app in which various actors need to share a cache so STM looks like the thing to use.

The cache has the semantics of a Scala Map. I see I could use TMap, but it seems like I could also just store the cache as an immutable Map inside a Ref.

What are the pros and cons of these two alternatives?


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