[GAP] Effect of using malloc for temporary allocations in GAP

Max Horn Max.Horn at math.uni-giessen.de
Tue Jan 3 17:11:40 GMT 2017


> On 03 Jan 2017, at 17:49, Dmitrii Pasechnik <dmitrii.pasechnik at cs.ox.ac.uk> wrote:
>> 

[...]
>> This question is "not even wrong" -- it simply doesn't make sense. You seem
>> to presuppose that "the heap" is a single memory region, which it is not.
>> The memory manager (e.g. libc's malloc) may and will allocate additional
>> pages to satisfy allocation requests.
>> 
>> To illustrate this by a hypothetical example: Suppose "the heap" so far
>> covers addresses 0x0 till 0x0FFFFF, and now we mmap some data, and it gets
>> mapped to addresses 0x10000 till 0x1FFFFF. Now somebody tries to malloc()
>> some data which does not fit in the existing hap anymore. Now libc extends
>> the heap, and quite possibly the new (additional) heap region will end up
>> right after our mmap'ed data. So we cannot grow that region contiguously
>> anymore.
> 
> This explanation appears to contradict the modern picture of contiguous
> virtual memory regions, which may well consist of non-contiguous
> physical pages (that's what e.g. vmalloc gives one in the Linux kernel).

No, it does not. I am talking about the address space visible to the process
(i.e. the virtual address space). How this is mapped to real space is
irrelevant to the problem I described above.

> Although probably I don't understand something here.

It seems so.

> 
>>>> With regards GASMAN. It is very fast, and I believe any replacement would be much slower. One of the biggest problems with HPC gap is that boehm is much slower.
>>>> 
>>>> Of course gasman has issues -- it can't support multithreading (hence can't be in hpcgap), is moving, requires continuous address space. But any replacement will, for gap's current usage, be I believe slower. Also much of gaps internals relies on features of gasman other allocators may not provide (like cheap access to the exact size of any allocation for example).
>>> 
>>> Why would GASMAN be problematic with multithreading? Surely, it will
>>> need a mechanism to pause all the other threads while collecting
>>> garbage, but apart from this, is there anything fundamental that
>>> makes it hard to port?
>> 
>> Pausing all threads to do GC is problematic on its own; for starters, all
>> memory allocations (which currently are blindly fast with GASMAN, and GAP
>> code kind of relies on that) would need to take a lock, which is expensive.
> 
> Why would allocations need a lock?

If a thread tries to allocate memory while a garbage collection is in
progress, it must sleep until the GC is complete. Hence the need for a lock.

Now before you feel tempted to say "why not put all other threads to sleep
as soon as a garbage collection starts", think about you would actually
implement that using POSIX pthread APIs. Don't say it unless you can
actually describe a race-free way to pull that off -- and of course without
using locks...


> Allocations may be queued and processed one by one.

I have no clue what you mean here. But it sounds like it would require locking
anyway.

> 
>> But let's ignore that for the moment (as perhaps there is a clever scheme to
>> avoid or reduce that overhead).
>> 
>> Then we still would have to teach GASMAN to inspect the stack and CPU
>> register contents (!) of all other running threads, as those could contain
>> references to objects. This is a very fundamental and major change. And
>> non-portable, too.  It makes up a considerable part of the complexity of
>> Boehm GC, I think.
>> 
> Oh, right, registers, this might be a pain indeed. One begins to understand why
> reference counting might be easier to deal with...

It is not. It has a big bunch of problems on its own.

> 
> [...]
> 
>> I guess e.g. Reimer would fit the bill, but he has a job already, and
>> AFAIK we have no money to pay him or anybody else to work on a new GC
>> anyway...
> 
> Although DFG forked out a huge sum for computer algebra,
> and some of it is reaching GAP-related German universities.

I don't think that they have asked for money for this (nor do I think they would have gotten it).

Dima, I really feel like I've spent the last couple emails trying to teach
you basics of virtual memory management, GC and concurrent GC. I think this
is not efficient. If you are really interested in these things, there are
online courses and books to learn. For example:

  http://gchandbook.org/

Bye,
Max
-- 
Prof. Dr. Max Horn
AG Algebra
Mathematisches Institut
Justus-Liebig-Universität Gießen
Arndtstraße 2
D-35392 Gießen
Tel: (+49) 641 99-32044
Fax: (+49) 641 99-32049
E-Mail: max.horn at math.uni-giessen.de




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