New Allocator Fails On >=4GB Requests

[c-blake]

proc myNewSeq*(capacity: uint64): seq[uint64] =
  result = newSeq[uint64](capacity)
#discard myNewSeq((1 shl 29) - 9)  # works
discard myNewSeq((1 shl 29) - 8)  # writes "out of memory"
#discard myNewSeq((1 shl 29) - 4)  # even worse rawNewObj ends up nil and zeroMem aborts

Personally, I allocate >4GB single seqs all the time. Even my phone has that much RAM.

I believe I tracked down the error to 32-bit arithmetic in alloc.nim:mappingSearch. { I just did a system call trace and noticed the roughly 4GB request translated to an mmap of 2**58 bytes. I then set a breakpoint at that crazy mmap and saw getBigChunk was the last call with a reasonable size request. Then reading the code for getBigChunk implicated mappingSearch. }

It looks like recent (Dec 7) code and that some 32-bit limitations are documented. So, this code failing on large request sizes may have been something that you are already vaguely aware of, but it seemed worth opening this issue.

[Araq]

TBH I didn't imagine anybody to use allocations bigger than 2GB. ;-)

[c-blake]

3 years ago I had access to a box with 1TB RAM and another 1TB swap space..room for literally 100s of seqs that might exceed 4GB all at the same time. ;-)

Anyway, https://github.com/mattconte/tlsf/ makes it seem like the changes to support 64-bits are pretty simple. (Just search for the code conditioned on TLSF_64BIT). That original TLSF paper was written in 2004 when 64-bit pointers were much more rare. For Nim it might be best to just use int64/uint64 unconditionally.

[Araq]

Want to give it a shot?

[c-blake]

Sure. I should be able to work on this sometime over the next week. Will send a PR when I have something.

[timotheecour]

FYI:

• tfragment_alloc.nim (which allocates 4GB) often makes appveyor fail with out of memory tfragment_alloc.nim (which allocates 4GB) often makes appveyor fail with out of memory #8509
• fix tfragment_alloc.nim (which allocates 4GB) often makes appveyor fail with out of memory #8509 disable 4GB allocating test on windows that crashed appveyor fix #8509 disable 4GB allocating test on windows that crashed appveyor #8510