Simon seems a bit busy right now. Can anyone else advise me on the basics of heap allocation in primops?
On Tue, Sep 25, 2018 at 1:42 PM, David Feuer <david.fe...@gmail.com> wrote: > Let's forget about allocate(). I can definitely handle this part in > C--. But I'm still lost in the macros and such. For example, I'm very > unclear on the differences among the ALLOC, HP_CHK, and MAYBE_GC > classes of macro. I can't find anything in the commentary, and the > source code documentation is very sparse. I'm okay with either of the > following approaches, but either way I need a bit more info. > > 1. First see if we need to allocate a StableName#. If so, check > whether GC would be required to allocate the StableName# (how?). If > so, drop the lock, run GC (how?) and start over. This looks cleanest > to me if it can be done easily. > > 2. First run the GC if we're low on memory (how?). Then if we need to > allocate a StableName#, we'll be sure to have room. > > > On Tue, Sep 25, 2018 at 6:25 AM, Simon Marlow <marlo...@gmail.com> wrote: >> You can do it unconditionally before taking the lock, or you can do it >> conditionally as long as you release the lock if the heap check fails. I >> think in the latter case there might not be a macro that allows this, but >> you could use the `allocate()` method for allocating memory (like >> newByteArray#) and then you could write a heap check like the MAYBE_GC() >> macro. Doing it unconditionally is easier and probably not a big performance >> hit, but note that you'll have to retreat Hp if you don't use the memory. >> >> Cheers >> Simon >> >> On Sat, 22 Sep 2018 at 13:08, David Feuer <david.fe...@gmail.com> wrote: >>> >>> How do I check if GC will be required, and how do I trigger it? Should I >>> perform the check unconditionally at the beginning of the operation so I >>> don't have to drop the lock, GC, then retake? I don't know the right ways to >>> deal with this stuff, and the macros are mostly undocumented. >>> >>> On Sep 22, 2018 3:53 AM, "Simon Marlow" <marlo...@gmail.com> wrote: >>> >>> Yes, the current implementation looks like it creates the object after >>> adding the entry to the StableName table and releasing the lock, which is >>> unsafe because another thread could read that same entry before the object >>> has been created. The easiest solution to that is to take and release the >>> lock in C-- in the right places instead of in the C lookupStableName() >>> function (you might need to make a separate version of lookupStableName() >>> that doesn't take the lock). >>> >>> Cheers >>> Simon >>> >>> >>> On Fri, 21 Sep 2018 at 12:53, David Feuer <david.fe...@gmail.com> wrote: >>>> >>>> It seems awkward to do it in C--, but maybe you can help me work out how. >>>> The allocation facilities definitely seem much nicer there, and allocating >>>> a >>>> small heap object in C feels like an abuse of the facilities we have there. >>>> The essential challenge, as I see it, is that we need the key to point to a >>>> valid stable name object by the time we drop the hash table lock. The >>>> process, as I imagine it: >>>> >>>> 1. Follow indirections, untag, choose the right generation. (All this is >>>> in C) >>>> 2. Take the appropriate hash table lock. (C) >>>> 3. Look up the key in the hash table (C). >>>> >>>> Now there's a branch. If we found the key, then we don't need to allocate >>>> an SNO. We just drop the lock and return. Otherwise >>>> >>>> 4. Allocate an SNO and set its info pointer (most easily done in C--). If >>>> this necessitates GC, we need to drop the lock first and might as well just >>>> go back to the very beginning afterwards. >>>> 5. Populate the SNO with its "hash value" (we can do this anywhere we >>>> like, I imagine). >>>> 6. Insert the key and SNO into the hash table and drop the hash table >>>> lock (C) >>>> 7. If necessary, insert the SNO into the remembered set (C) >>>> >>>> How would you recommend structuring this back-and-forth? >>>> >>>> On Fri, Sep 21, 2018, 3:19 AM Simon Marlow <marlo...@gmail.com> wrote: >>>>> >>>>> I'm a bit sceptical that you need to allocate a heap object in C instead >>>>> of C--, but still, here's an example: >>>>> https://phabricator.haskell.org/diffusion/GHC/browse/master/rts%2FThreads.c$258-261 >>>>> >>>>> It's slightly less efficient to do this in C than C--, because >>>>> `allocate()` is slower than allocating by bumping `Hp`. >>>>> >>>>> On Mon, 17 Sep 2018 at 21:25, David Feuer <david.fe...@gmail.com> wrote: >>>>>> >>>>>> How can I allocate a heap object in C code in the rts? I've only seen >>>>>> heap objects allocated in C--, and doing that here would be lousy for >>>>>> performance and worse for clarity. >>>>>> >>>>>> David >>> >>> >> _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs