On 23/12/09 07:55, Herk, Robert van wrote:
Hi all,
I stumbled upon something odd with respect to arrays. I know about GHC not
doing card marking and traversing whole arrays one each GC for each array with
alterations, but still I don't understand the behaviour.
The situation is like this. I am building a compiler. This compiler manipulates
a large graph (for optimizations, etc.). This graph is in memory. As the graph
is vast, we did a lot of effort to optimize access to it. The nodes in the
graph are IORefs to some data structure, and this data structure contains its
edges.
Each node stores its edges in buckets. This is because edges have different
types (some are control flow links, others are other types of dependencies).
Most of the graph manipulations only traverse over one type of edge, so we
figured it would be faster to store the edges in buckets to support such
queries. Inside the buckets, there are Data.Maps containing the actual edges
for that bucket. The keys in this map are the target nodes of that edges, which
are IORefsOrds, which are pairs of a unique integers and a IORef, such that
they can be ordered and used as keys in a Map. The values are lists of edges to
that target.
The weird thing is in the buckets. Per node, all buckets are stored in an
array. We gave each edge type an integer key.
And we use that key as array index to determine the bucket. I've tried
implementing this array in two ways:
1) Each node contains a immutable array with IORefs. The IORefs contain the
actual Data.Maps for the buckets. So, for instance, initializing a node looks
something like
import Data.Array
uBucket = 8 //There are 8 buckets because there are 8 types of edges.
emptyEdges =
do buckets<- sequence ( take uBucket $ repeat (newIORef Map.empty) )
let myArray = listArray (0, 7) buckets
return myArray
So in this solution we have an extra layer of indirection, namely the IORefs,
but the array is immutable. Because when the edges change for a particular
node, we can write directly into the IORef and leave the array untouched.
2) Each node contains a mutable array that contains the Data.Maps directly. So,
for instance, initializing a node looks something like:
import Data.Array.IO
emptyEdges =
do myArray<- newArray (0, 7) Map.empty
return $! myArray
Of course, one expect 2 to be quickest. However, it turns out that for 2, the application
is spending much more time in GC, and __even without GC__ the application is still
slower! I find both things rather weird: I know that for huge arrays I am expected to
suffer from the missing card-marking "bug", but my array sizes are only 8.
If you have lots of small mutable arrays, that could be a problem. We
implement the write barrier differently for arrays vs. IORefs: an IORef
is only placed in the remembered set (and hence scanned during minor GC)
if it is modified. Mutable arrays on the other hand are always in the
remembered set, but they are marked either clean or dirty. There's a
tradeoff here: the approach we take for arrays means that writes are
quicker, but each array has to be checked in a minor GC. It works well
when arrays are few and large, but less well when they are numerous and
small.
Incedentally, I recently committed changes to add card marking to
mutable arrays. In your case it will probably hurt performance rather
than improve it, though, since you won't benefit from the card table and
the write barrier is more expensive.
One solution would be to have a new array type optimised for this case.
Alternatively we could experiment with the more expensive write
barrier and see how much it hurts the large-array cases.
I'd like to verify that this is really the problem - any chance you can
give us a self-contained snapshot of your code that we can use as a
benchmark?
Cheers,
Simon
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