[infinispan-dev] CHM or CHMv8?

[Dan Berindei]

Right. If we have anywhere a map that's initialized from a single thread
and then accessed only for reading from many threads, it probably makes
sense to use a HashMap and wrap it in an UnmodifiableMap. But if it can be
written from multiple threads as well, I think we should use a CHMV8.

BTW, the HashMap implementation in OpenJDK 1.7 seems to have some
anti-collision features (a VM-dependent hash code generator for Strings),
but our version of CHMV8 doesn't. Perhaps we need to upgrade to the latest
CHMV8 version?



On Fri, Apr 19, 2013 at 4:32 PM, David M. Lloyd <david.lloyd at redhat.com>wrote:

> On 04/19/2013 08:22 AM, Sanne Grinovero wrote:
> > On 19 April 2013 13:52, David M. Lloyd <david.lloyd at redhat.com> wrote:
> >> On 04/19/2013 05:17 AM, Sanne Grinovero wrote:
> >>> On 19 April 2013 11:10, Dan Berindei <dan.berindei at gmail.com> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On Fri, Apr 19, 2013 at 12:58 PM, Sanne Grinovero <
> sanne at infinispan.org>
> >>>> wrote:
> >>>>>
> >>>>> On 19 April 2013 10:37, Dan Berindei <dan.berindei at gmail.com> wrote:
> >>>>>> Testing mixed read/write performance with capacity 100000, keys
> 300000,
> >>>>>> concurrency level 32, threads 12, read:write ratio 99:1
> >>>>>> Container CHM           Ops/s 5178894.77  Gets/s 5127105.82  Puts/s
> >>>>>> 51788.95  HitRatio      86.23  Size     177848  stdDev   60896.42
> >>>>>> Container CHMV8         Ops/s 5768824.37  Gets/s 5711136.13  Puts/s
> >>>>>> 57688.24  HitRatio      84.72  Size     171964  stdDev   60249.99
> >>>>>
> >>>>> Nice, thanks.
> >>>>>>
> >>>>>> The test is probably limited by the 1% writes, but I think it does
> show
> >>>>>> that
> >>>>>> reads in CHMV8 are not slower than reads in OpenJDK7's CHM.
> >>>>>> I haven't measured it, but the memory footprint should also be
> better,
> >>>>>> because it doesn't use segments any more.
> >>>>>>
> >>>>>> AFAIK the memoryCHMV8 also uses copy-on-write at the bucket level,
> but
> >>>>>> we
> >>>>>> could definitely do a pure read test with a HashMap to see how big
> the
> >>>>>> performance difference is.
> >>>>>
> >>>>> By copy-on-write I didn't mean on the single elements, but on the
> >>>>> whole map instance:
> >>>>>
> >>>>> private volatile HashMap configuration;
> >>>>>
> >>>>> synchronized addConfigurationProperty(String, String) {
> >>>>>        HashMap newcopy = new HashMap( configuration ):
> >>>>>        newcopy.put(..);
> >>>>>        configuration = newcopy;
> >>>>> }
> >>>>>
> >>>>> Of course that is never going to scale for writes, but if writes stop
> >>>>> at runtime after all services are started I would expect that the
> >>>>> simplicity of the non-threadsafe HashMap should have some benefit
> over
> >>>>> CHM{whatever}, or it would have been removed already?
> >>>>>
> >>>>
> >>>> Right, we should be able to tell whether that's worth doing with a
> pure read
> >>>> test with a CHMV8 and a HashMap :)
> >>>
> >>> IFF you find out CHMV8 is as good as HashMap for read only, you have
> >>> two options:
> >>>    - ask the JDK team to drop the HashMap code as it's no longer needed
> >>>    - fix your benchmark :-P
> >>>
> >>> In other words, I'd consider it highly surprising and suspicious
> >>> (still interesting though!)
> >>
> >> It's not as surprising as you think.  On x86, volatile reads are the
> >> same as regular reads (not counting some possible reordering magic).  So
> >> if a CHM read is a hash, an array access, and a list traversal, and so
> >> is HM (and I believe this is true though I'd have to review the code
> >> again to be sure), I'd expect very similar execution performance on
> >> read.  I think some of the anti-collision features in V8 might come into
> >> play under some circumstances though which might affect performance in a
> >> negative way (wrt the constant big-O component) but overall in a
> >> positive way (by turning the linear big-O component into a logarithmic
> one).
> >
> > Thanks David. I know about the cost of a volatile read, what I'm
> referring to
> > is that I would expect the non-concurrent Maps to generally contain some
> > simpler code than a conccurrent one. If this was not the case,
> > why would any JDK team maintain two different implementations?
> > That's why I would consider it surprising if it turned out that the
> CHMV8 was
> > superior over a regular one on all fronts: there certainly is some
> > scenario in which the regular one would be a more appropriate choice,
> > which directly proofs that blindly replacing all usages in a large
> project
> > is not optimal. Of course, it might be close to optimal..
>
> You are right, it is not superior on all fronts.  It is definitely
> similar in terms of read, but writes will have a substantially higher
> cost, involving (at the very least) multiple volatile writes which are
> orders of magnitude more expensive than normal writes (on Intel they
> have the costly impact of memory fence instructions).  So I don't think
> anyone will want to drop HashMap any time soon. :-)
>
> --
> - DML
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