Re: [Bug-gnubg] Cache question

[Jonathan Kinsey]

The cache uses a hash function to map the position onto a cache entry, there is
every chance of several positions mapping to the same position (the cache stores
2 positions per entry to minimise this).

The reason things get better even when the cache is bigger than the total number
of evaluations is there will be less collisions in the cache. There is a
#define you can set and then a command to get the cache statistics (lookups, hits).

It might be possible to improve the hash function (GetHashKey), but it does seem
to do quite a good job.

Jon

Christian Anthon wrote:
> Hi Jon,
>
> I assume you tested 2 ply. 4ply will continue to get faster with
> increased cache sizes. At least until I ran out of my 2GB of memory.
> But why do we need this large a cache when we only need to store
> 21+21^2+21^3+21^4 = 204204 positions to do a full 4ply, which is <
> 2^17 entries taking in to account that the current cache store two
> positions per entry. It doesn't seem very rational.
>
> Christian.
>
>
> On Thu, Sep 3, 2009 at 12:27 PM, Jonathan Kinsey wrote:
>> As the cache gets larger it riches a natural plateau, in my initial tests
>> this
>> was at the current gui limit (4gb entries), I was looking at Ingo's results
>> just
>> yesterday and there was little (or no) improvement above 2^15 entries. The
>> settings and position types will be a factor.
>>
>> It's quite easy to try out, just use the "set cache" command, you need to
>> enter
>> the number of entries in powers of 2 though, so the current maximum (2^22)
>> would be:
>> set cache 4194304
>> just times this by 2 to increase the cache (obviously this doubles the
>> memory
>> usage as well).
>>
>> I'd be interested in your results as it might be worth changing the
>> minimum/maximum and default settings displayed in the gui.
>>
>> Jon
>>
>> Michael Depreli wrote:
>>>>From my tests the extra speed from using max cache although not huge
>>> was worthwhile.
>>> Given like you say>2gb is the norm (I have 4gb) would it be worth
>>> having an even larger cache available in gnubg?
>>>
>>>
>>> ------------------------------------------------------------------------
>>> From: address@hidden
>>> To: address@hidden
>>> CC: address@hidden
>>> Subject: Re: [Bug-gnubg] Cache question
>>> Date: Thu, 3 Sep 2009 09:37:15 +0000
>>>
>>> The only reason is if the memory usage has any impact for you (running
>>> several
>>> copies at the same time for example), it's getting a lot less likely
>>> that this
>>> is the case with>2gb becoming the norm on new pcs.
>>>
>>> You might find that there is little difference in speed between the
>>> maximum and
>>> one-down setting (and this would save you 80mb of memory).
>>>
>>> Jon
>>>
>>> Michael Depreli wrote:
>>>> I ran some brief tests using rollouts with different cache settings and
>>>> larger cache produced faster results (not linear).
>>>> Are there any known issues (bugs) running gnubg with cache set to max
>>>> for evals and rollouts for plies up to 2?
>>>> If not are there any reasons to not set cache to max?
>>>>
>>>> Michael
>>>>
>>>>
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