GHC memory usage when typechecking from source vs. loading ModIfaces

Fri Mar 28 10:12:22 UTC 2025

PUBLIC

Just to add that I get the same "equalizing" behaviour (but in a more "natural" way) if instead of deepseq-ing the ModuleGraph upfront, I just call `hugInstancesBelow` before processing each module. So that's definitely one source of extra memory usage. I wonder if it would be possible to rebuild the ModuleGraph periodically (similar to the ModDetails dehydration), or if there are references to it stored all over the place from `HscEnv`s scattered around in closures etc. (basically the same problem the HPT had before it was made into a mutable reference).

-----Original Message-----
From: ghc-devs <ghc-devs-bounces at haskell.org> On Behalf Of Erdi, Gergo via ghc-devs
Sent: Friday, March 28, 2025 4:49 PM
To: Matthew Pickering <matthewtpickering at gmail.com>; GHC Devs <ghc-devs at haskell.org>
Cc: ÉRDI Gergő <gergo at erdi.hu>; Montelatici, Raphael Laurent <Raphael.Montelatici at sc.com>; Dijkstra, Atze <Atze.Dijkstra at sc.com>
Subject: [External] Re: GHC memory usage when typechecking from source vs. loading ModIfaces

Hi,

Unfortunately, I am forced to return to this problem. Everything below is now in the context of GHC 9.12 plus the mutable HPT patch backported.

My test case is typechecking a tree of 2294 modules that form the transitive closure of a single module's dependencies, all in a single process. I have done this typechecking three times, here's what `+RTS -s -RTS` reports for max residency:

* "cold": With no on-disk `ModIface` files, i.e. from scratch: 537 MB

* "cold-top": With all `ModIface`s already on disk, except for the
  single top-level module: 302 MB

* "warm": With all `ModIface`s already on disk: 211 MB

So my stupidly naive question is, why is the "cold" case also not 302 MB?

In earlier discussion, `ModDetails` unfolding has come up. Dehydrating `ModDetails` in the HPT all the time is disastrous for runtime, but based on this model I would expect to see improvements from dehydrating "every now and then". So I tried a stupid simple example where after every 100th typechecked module, I run this function on the topologically sorted list of modules processed so far:

```
dehydrateHpt :: HscEnv -> [ModuleName] -> IO () dehydrateHpt hsc_env mods = do
    let HPT{ table = hptr } = hsc_HPT hsc_env
    hpt <- readIORef hptr
    for_ mods \mod -> for_ (lookupUDFM hpt mod) \(HomeModInfo iface _details _linkable) -> do
        !details <- initModDetails hsc_env iface
        pure ()
```

Buuut the max residency is still 534 MB (see "cold-dehydrate"); in fact, the profile looks exactly the same.

Speaking of the profile, in the "cold" case I see a lot of steadily increasing heap usage from the `ModuleGraph`. I could see this happening if typechecking from scratch involves more `modulesUnder` calls which in turn force more and more of the `ModuleGraph`. If so, then maybe this could be worked around by repeatedly remaking the `ModuleGraph` just like I remake the `ModDetails` above. I tried getting rid of this effect by `deepseq`'ing the `ModuleGraph` at the start, with the idea being that this should "equalize" the three scenarios if this really is a substantial source of extra memory usage. This pushes up the warm case's memory usage to 381 MB, which is promising, but I still see a `Word64Map` that is steadily increasing in the "cold-force-modulegraph" case and contributes a lot to the memory usage. Unfortunately, I don't know where that `Word64Map` is (it could be any `Unique`-keyed environment...).

So I am now stuck at this point. To spell out my goal explicitly, I would like to typecheck one module after another and not keep anything more in memory around than if I loaded them from `ModIface` files.

Thanks,
        Gergo

p.s.: I couldn't find a way in the EventLog output HTML to turn event markers on/off or filter them, so to avoid covering the whole graph with gray lines, I mark only every 100th module.

From: Matthew Pickering <matthewtpickering at gmail.com>
Sent: Wednesday, February 12, 2025 7:08 PM
To: ÉRDI Gergő <gergo at erdi.hu>
Cc: Erdi, Gergo <Gergo.Erdi at sc.com>; Zubin Duggal <zubin at well-typed.com>; Montelatici, Raphael Laurent <Raphael.Montelatici at sc.com>; GHC Devs <ghc-devs at haskell.org>
Subject: [External] Re: GHC memory usage when typechecking from source vs. loading ModIfaces

You do also raise a good point about rehydration costs.

In oneshot mode, you are basically rehydrating the entire transitive closure of each module when you compile it, which obviously results in a large amount of repeated work. This is why people are investigating ideas of a persistent worker to at least avoid rehydrating all external dependencies as well.

On Mon, Feb 10, 2025 at 12:13 PM Matthew Pickering <mailto:matthewtpickering at gmail.com> wrote:
Sure, you can remove them once you are sure they are not used anymore.

For clients like `GHCi` that doesn't work obviously as they can be used at any point in the future but for a batch compiler it would be fine.

On Mon, Feb 10, 2025 at 11:56 AM ÉRDI Gergő <mailto:gergo at erdi.hu> wrote:
On Mon, 10 Feb 2025, Matthew Pickering wrote:

> I wonder if you have got your condition the wrong way around.
>
> The only "safe" time to perform rehydration is AFTER the point it can
> never be used again.
>
> If you rehydrate it just before it is used then you will repeat work
> which has already been done. If you do this, you will always have a
> trade-off between space used and runtime.

Oops. Yes, I have misunderstood the idea. I thought the idea was that after loading a given module into the HPT, its ModDetails would start out small (because of laziness) and then keep growing in size as more and more of it are traversed, and thus forced, during the typechecking of its dependees, so at some point we would want to reset that into the small initial representation as created by initModDetails.

But if the idea is that I should rehydrate modules when they can't be used anymore, then that brings up the question why even do that, instead of straight removing the HomeModInfos from the HPT?

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