Stable pointers and hash table performance

[Eyal Lotem]

Hey,

Background
----------------
I built a hash table in C:  https://github.com/Peaker/small_hash

According to a few simple benchmarks (mainly, 10 million insertions), my C
hash table is much faster than any Haskell data structure I tried (e.g: 8
times faster than IntMap).
You can run the C benchmark (in small_hash) via: "make && ./benchmark
10000000".

The other benchmarks are at: https://github.com/Peaker/hashtable_benchmarks
Benchmarks results on my hardware: http://hpaste.org/81902

I was hoping to get that fast mapping goodness to Haskell, by building an
FFI to it:
https://github.com/Peaker/small_hash_hs

In order to do FFI bindings to a C data structure, that can contain
arbitrary Haskell values, I must create a lot of StablePtrs (for each
Haskell value stored in the C hash table).

Problem 1:
-------------
Disappointingly, the FFI bindings were dozens of times slower(!) than the C
code.

When using +RTS -H200M the speed is about 10 times slower(!) than the C
code.
oprofile shows that the culprit is the Stable Ptr code.

Digging in, I found that stable ptrs and stable names shared a table,
despite having quite different characteristics:
 * Stable Names aren't considered roots by GC, ptrs are.
 * Making two stable ptrs for the same Haskell value doesn't really require
them to unify -- so there's no need for the hash table or the "refs"
counter for stable ptrs.

My Changes:
-----------------
I split the Stable Names into their own table (which allowed removing the
"refs" field from them).
Stable Ptrs no longer use a hash table to unify, don't need "refs", or
"sn_obj", or "old".

This made Stable Ptrs cost just 1 C ptr in the table and really cheap to
create/destroy.

The commits are here:
https://github.com/Peaker/ghc/commits/master

They are not polished (I didn't thoroughly review myself yet, didn't run
the test suite, fix out-of-date comments involved) and not ready for
integration yet.

Result:
---------
The hash table benchmark via the FFI is now only ~2 times slower than the C
data structure and much faster than any other mapping structure for that
particular benchmark (10 million insertions of identity integers,
sequentially). A lot of the cost here is due to fine-grained allocation, as
the C code does bulk allocation. I shall improve this benchmark later, and
I believe it will tighten the gap some more.

Problem 2:
--------------
http://hackage.haskell.org/trac/ghc/ticket/7670

Every minor GC iterates a whole lot of stable ptrs. This means that with
the default heap size, runtime is about 8 times longer (than with a 200M
heap size).

To avoid this, I'd like to split the stable ptrs table into generations.

To do this, I need to know which of the pointed objects were promoted and
to what generation, so I can promote my stable ptrs accordingly.
It seems like the correct time to do this is in the updateStablePtrTable
(now named "updateStableTables")

This is the part I need help with.
How do I know if an object was promoted to a higher generation when I
traverse the objects?

Thanks!
Eyal
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