[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
[Chicken-hackers] Heap size calculation in rereclaim2
From: |
Peter Bex |
Subject: |
[Chicken-hackers] Heap size calculation in rereclaim2 |
Date: |
Tue, 13 Oct 2015 20:39:26 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Hi hackers,
This morning on IRC we were discussing the random breakage on Salmonella,
and Evan mentioned that he noticed that the crashes he observed were all
at the start of a program. This made me think that perhaps the problem
is in how literal decoding is happening and allocated into the heap,
and I discovered https://bugs.call-cc.org/ticket/1221
I've done some debugging and it makes sense why it's failing:
C_rereclaim2() gets invoked with the size demanded by the literals
in the toplevel. Just look at C_toplevel in the code generated by
the example program.
The example program demands a total of 1011010 words which is slightly
less than 8MiB on a 64-bit machine. When starting the program with a
smallish heap, it'll immediately invoke C_rereclaim2 as follows:
C_rereclaim2(1011010 * sizeof(C_word), 1);
Because double_plus (the second '1' argument) is set, it will trigger
this condition at the start of C_rereclaim2:
if(double_plus) size = heap_size * 2 + size;
So the new heap size is 2 * 1M + 8M = 10M if the initial heap was 1M.
Then, it'll do some range checks and debug output, and then it splits
the heap in two halves:
heap_size = size; /* Total heap size of the two halves... */
size /= 2; /* ...each half is this big */
Unfortunately, this means the new heap will be 5M, which is too small
to hold 8M of data!
So, the cause is simple, but I'm not so sure about the fix. I was
tempted to just change it to:
if(double_plus) size = heap_size * 2 + size * 2;
But then I looked at the other invocations, and started to wonder
what the meaning is of the size and double_plus arguments. In some
places, "size" is passed as the total intended size of the new heap,
while in other places, it is passed as the required _additional_ size
for the heap(!). I think if this is the case, double_plus is always
set but I'm not 100% sure.
Am I correct in thinking that double_plus is misnamed and should really
be called "relative_size" or something? Felix: Do you remember the
original meaning of these two parameters, how are they to be used?
Cheers,
Peter
signature.asc
Description: Digital signature
- [Chicken-hackers] Heap size calculation in rereclaim2,
Peter Bex <=
- Message not available
- Re: [Chicken-hackers] Heap size calculation in rereclaim2, Peter Bex, 2015/10/14
- Re: [Chicken-hackers] Heap size calculation in rereclaim2, Jörg F. Wittenberger, 2015/10/14
- Re: [Chicken-hackers] Heap size calculation in rereclaim2, Peter Bex, 2015/10/14
- Re: [Chicken-hackers] Heap size calculation in rereclaim2, Peter Bex, 2015/10/17
- Re: [Chicken-hackers] Heap size calculation in rereclaim2, Peter Bex, 2015/10/24
- Re: [Chicken-hackers] Heap size calculation in rereclaim2, Evan Hanson, 2015/10/25
- Re: [Chicken-hackers] Heap size calculation in rereclaim2, felix . winkelmann, 2015/10/25