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From "Aleksey Shipilev" <aleksey.shipi...@gmail.com>
Subject Re: My first submission! java.lang.ThreadLocal
Date Fri, 06 Jun 2008 18:49:48 GMT
Thanks for a thoughtful reply, Bob!
Let's get the code to the trunk.

Tim, Mark, Nathan, should we acquire a gold ticket to modify java.lang.Thread?

Thanks,
Aleksey.

On Fri, Jun 6, 2008 at 10:36 PM, Bob Lee <crazybob@crazybob.org> wrote:
> On Fri, Jun 6, 2008 at 10:09 AM, Aleksey Shipilev <
> aleksey.shipilev@gmail.com> wrote:
>
>> I'm not saying that your implementation is bad. On contrary, your
>> implementation is much faster than original Harmony one. What I'm
>> saying is, ThreadLocalBench is still 3x slower on Harmony/DRLVM than
>> on Sun 1.6.0_05. That might be connected with poor codegeneration on
>> Harmony JIT, some disabled optimizations there, etc. I don't know what
>> the real cause is. That's the question to answer.
>>
>
> It sounds like you're comparing DRLVM's JIT compiler to Sun's. I'm not at
> all surprised by the outcome--Sun has quite a head start.
>
>
>> Leaving Harmony/DRLVM aside, it's wonderful that your implementation
>> is going neck-to-neck with Sun's on another VM. So, why don't to
>> experiment and beat them? :)
>>
>
> Sun's impl was finely tuned by Doug Lea, BTW. I do beat the RI in 3 tests,
> they beat me in 2. When I say one impl beats another, we're really talking
> about a few ns in either direction; there's really no point in comparing. I
> think I've prioritized the correct use cases. I'm a little slower in tests
> which instantiate and throw away millions of thread local variables. I don't
> think that's the norm.
>
> Besides raw throughput, I also beat the RI in a few more areas: more
> agressive clean up, smaller memory footprint, and fewer allocations and
> garbage collections.
>
> I do plan to contribute it to the OpenJDK.
>
>
>> From this point-of-view, I raised the
>> question on rationale of having open-addressed + linear-probed hashmap
>> instead of, say, chaining. Taking away caching opportunities, Knuth
>> had analyzed the average lookup count for different probing schemes,
>> and linear probing performed the baddest.
>
>
> That doesn't apply here. Knuth used English words for keys. We generate well
> dispersed hash codes. In practice, this implementation almost never actually
> probes. When it does probe, it probes the same area of memory.
>
>
>> Actually, we had recently
>> moved IdentityHashMap from the exactly the same addressing to chaining
>> [1], and results are pretty good. There's additional +80% on
>> ThreadLocalBench and I expect we can get them on your implementation
>> too!
>
>
> In general, you should favor open addressing over chaining: less
> indirection, fewer allocations, smaller memory footprint, better memory
> locality (if you use linear probing). My guess is that IdentityHashMap just
> had a flawed impl of open addressing--it's very easy to get something wrong.
>
> I could have used quadratic probing, double hashing, or cuckoo hashing, but
> all of those are aimed at avoiding clustering. I generate well dispersed
> hash codes in the first place, so they would have just added more complexity
> and reduced memory locality with no benefit.
>
> Obviously, if you're using externally provided hash codes, you have to use
> chaining, but that's not the case here.
>
> Bob
>

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