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Re: multithreaded cache?

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On 19.05.2012 00:31, Eric Sosman wrote:
> On 5/18/2012 5:45 PM, Robert Klemme wrote:
>> On 18.05.2012 20:42, Silvio Bierman wrote:
>>> On 05/17/2012 11:54 AM, Robert Klemme wrote:
>>
>>>> I provide a variant of Silvio's, Eric's and Daniel's solution which
>>>> should yield higher throughput because it works without read write
>>>> locking. You can find it as gist in case the code is garbled in the
>>>> newsgroup posting:
>>>> https://gist.github.com/2717818
>>
>>> I think you have as many locks as I suggested (being one)? My initial
>>> implementations of something like this used a plain map with an extra
>>> lock but later cases used the by then available ConcurrentHashMap as
>>> well, making one lock redundant.
>>
>> You didn't show it here, did you? I can's seem to find it in the thread.
>> Note that CHM does also do synchronization. I am not sure from your
>> statement what exact locking scheme you apply. There does seem to be one
>> difference though: I my version the second lock goes away after the
>> value has been computed so there is only the sync of CHM left.
>
> It seems to me that if N threads query the same key at about
> the same time, they may all miss the map and go off to perform
> the slow computation. If "slow" is large compared to the cost of
> a lock-release pair (and if it weren't, why cache?), the tradeoff
> seems suspect.
>
> Also, different threads may wind up using different value
> instances. If the cache is purely a convenience for a value-only
> object that may be all right, but it's not all right if the values
> are supposed to be singletons.
>
> Finally, there's more than a whiff of the double-checked locking
> antipattern about what you're doing with the `here' flag. I'm not
> absolutely sure what you've got is in fact DCL (hence, wrong), but
> I'm also not absolutely sure it's DCL-free. Before using the pattern
> in any important way, I'd want to check with a major-league guru,
> just as "due diligence."

There is no double checked locking going on - neither in LazyCache 
itself nor in the retriever instance.
http://en.wikipedia.org/wiki/Double-checked_locking

Daniel has explained all the details already.  I'd just add that 
creation of multiple retriever objects is the price you pay for 
increased concurrency due to CHM.  But only one of them is ever going to 
be used per key.  (Documenting this is one of the tasks of the assert 
Daniel questioned earlier.)

I think rw-locking is an inferior technique compared to CHM in this case 
because in case of cache miss you cannot escalate a r-lock to a w-lock 
(to avoid deadlocks) and hence need to release the r-lock, acquire the 
w-lock, *and check again*.  In other words you have two more 
synchronization points with memory barriers, scheduling effects etc.

In case of cache hit you might still suffer throughput because of thread 
starvation caused by all lookups for values missing from the cache might 
be blocked for indefinite time when using an unfair rw-lock.  Using a 
fair rw-lock on the other hand is more expensive and still has the 
downside of a global lock which affects the complete range of keys.

CHM improves this by partitioning value space and only lock individual 
partitions for atomic operations.  These partitions are completely 
independent from a locking perspective.  That's the main advantage of CHM.

Kind regards

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/

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Thread

multithreaded cache? bugbear <bugbear@trim_papermule.co.uk_trim> - 2012-05-15 10:14 +0100
  Re: multithreaded cache? Silvio Bierman <silvio@moc.com> - 2012-05-15 11:41 +0200
    Re: multithreaded cache? Roedy Green <see_website@mindprod.com.invalid> - 2012-05-15 15:57 -0700
      Re: multithreaded cache? Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-05-15 16:13 -0700
        Re: multithreaded cache? Silvio Bierman <silvio@moc.com> - 2012-05-16 01:22 +0200
      Re: multithreaded cache? Silvio Bierman <silvio@moc.com> - 2012-05-16 01:19 +0200
        Re: multithreaded cache? Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-05-15 20:26 -0400
          Re: multithreaded cache? Arved Sandstrom <asandstrom3minus1@eastlink.ca> - 2012-05-15 21:41 -0300
          Re: multithreaded cache? Silvio Bierman <silvio@moc.com> - 2012-05-16 09:26 +0200
            Re: multithreaded cache? "Mike Schilling" <mscottschilling@hotmail.com> - 2012-05-26 14:54 -0700
        Re: multithreaded cache? Roedy Green <see_website@mindprod.com.invalid> - 2012-05-16 22:03 -0700
          Re: multithreaded cache? Silvio Bierman <silvio@moc.com> - 2012-05-21 15:25 +0200
  Re: multithreaded cache? Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-05-15 08:22 -0400
  Re: multithreaded cache? Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-05-15 09:56 -0700
  Re: multithreaded cache? bugbear <bugbear@trim_papermule.co.uk_trim> - 2012-05-16 14:33 +0100
  Re: multithreaded cache? Robert Klemme <shortcutter@googlemail.com> - 2012-05-17 11:54 +0200
    Re: multithreaded cache? Robert Klemme <shortcutter@googlemail.com> - 2012-05-17 12:06 +0200
      Re: multithreaded cache? Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-05-17 09:51 -0700
        Re: multithreaded cache? Robert Klemme <shortcutter@googlemail.com> - 2012-05-17 20:48 +0200
      Re: multithreaded cache? Sebastian <sebastian@undisclosed.invalid> - 2012-05-20 01:35 +0200
        Re: multithreaded cache? Sebastian <sebastian@undisclosed.invalid> - 2012-05-20 01:48 +0200
          Re: multithreaded cache? Lew <noone@lewscanon.com> - 2012-05-19 18:11 -0700
    Re: multithreaded cache? Silvio Bierman <silvio@moc.com> - 2012-05-18 20:42 +0200
      Re: multithreaded cache? Robert Klemme <shortcutter@googlemail.com> - 2012-05-18 23:45 +0200
        Re: multithreaded cache? Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-05-18 18:31 -0400
          Re: multithreaded cache? Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-05-18 22:15 -0700
            Re: multithreaded cache? Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-05-19 07:09 -0400
          Re: multithreaded cache? Robert Klemme <shortcutter@googlemail.com> - 2012-05-19 12:33 +0200
            Re: multithreaded cache? Lew <noone@lewscanon.com> - 2012-05-19 14:24 -0700
              Re: multithreaded cache? Robert Klemme <shortcutter@googlemail.com> - 2012-05-21 21:15 +0200
                Re: multithreaded cache? Lew <noone@lewscanon.com> - 2012-05-23 23:11 -0700
                Re: multithreaded cache? Robert Klemme <shortcutter@googlemail.com> - 2012-05-24 23:05 +0200
                Re: multithreaded cache? Gene Wirchenko <genew@ocis.net> - 2012-05-24 20:11 -0700
                Re: multithreaded cache? Robert Klemme <shortcutter@googlemail.com> - 2012-05-25 17:42 +0200
                Re: multithreaded cache? Arved Sandstrom <asandstrom3minus1@eastlink.ca> - 2012-05-25 07:09 -0300
        Re: multithreaded cache? Silvio Bierman <silvio@moc.com> - 2012-05-21 15:21 +0200
  Re: multithreaded cache? Kevin McMurtrie <mcmurtrie@pixelmemory.us> - 2012-05-24 23:22 -0700
    Re: multithreaded cache? Robert Klemme <shortcutter@googlemail.com> - 2012-05-25 17:36 +0200
      Re: multithreaded cache? Kevin McMurtrie <mcmurtrie@pixelmemory.us> - 2012-06-02 12:27 -0700
        Re: multithreaded cache? Robert Klemme <shortcutter@googlemail.com> - 2012-06-02 21:54 +0200

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