Re: Understanding the -A and the -H flags

On 27/02/2012 17:34, Johan Tibell wrote:

Hi Simon,

On Mon, Feb 27, 2012 at 12:25 AM, Simon Marlow wrote:

...

"Think of -Hsize as a variable -A option. It says: I want to use at least size bytes, so use whatever is left over to increase the -A value."

Doesn't that describe exactly what it means?

Maybe. Let me start with the mental model I approach this with: the allocation area (i.e. the nursery) should have a size in the order of megabytes, often around the size of the L2 cache.

Ah, so I see where your confusion arises - this assumption is not true in general. Just discard the assumption, and I think everything will make more sense. Picking a size for -A around the L2 cache is often a good idea, but not always. GHC defaults to -A512K, but programs that benefit from much larger sizes are quite common. For more about the tradeoff, see my SO answer here: http://stackoverflow.com/questions/3171922/ghcs-rts-options-for-garbage-coll...

Given this model, I read the above as:

* if you set e.g. -H1G, you'll get an allocation area which is in the order of 1Gb large. That makes no sense to me.

Right - see above. In fact there's no problem with a 1GB nursery.

* The suggested size of the total heap (-H) has something to do with the size of the allocation area (-A). This makes no sense to me either.

So either I do understand what -H does, but it makes no sense to me, or I don't understand what -H does, but what it does makes sense.

Perhaps the confusion lies in the phrase "left over." Left over from what?

Left over after the memory required by the non-nursery parts of the heap has been deducted. Cheers, Simon