Re: [9fans] Re: _threadmalloc() size>100000000; shouldn't be totalmalloc?

[adr <adr@SDF.ORG>]

On Tue, 28 Jun 2022, Dan Cross wrote:
>>> [...]
>>> void*
>>> _threadmalloc(long size, int z)
>>> {
>>>       void *m;
>>>
>>>       m = malloc(size);
>>>       if (m == nil)
>>>               sysfatal("Malloc of size %ld failed: %r", size);
>>>       setmalloctag(m, getcallerpc(&size));
>>>       totalmalloc += size;
>>>       if (size > 100000000) {
>>>               fprint(2, "Malloc of size %ld, total %ld\n", size,
>>> totalmalloc);
>>>               abort();
>>>       }
>>>       if (z)
>>>               memset(m, 0, size);
>>>       return m;
>>> }
>>> [...]
>>>
>>> Shouldn't the if statement test the size of totalmalloc before the
>>> abort? That size, 100M for internal allocations? It has to be
>>> totalmalloc, doesn't it? If not this if statement should be after
>>> testing the success of the malloc. Am I missing something?
>
> Note that the `if` statement doesn't test the size of *totalmalloc*, but
> just `size`.  `totalsize` is only incidentally printed in the output if `size`
> exceeds 100 MB: that is, if a single allocation is larger than 100MB.

I know, what called my attention is that size wasn't tested before
calling malloc, so the first thought I had was that the code had
a typo and it should be testing totalmalloc instead.

>> I mean, I think using libthread more like using small channels and thread's stack sizes, but:
>>
>> Why put a limit here to size when totalmalloc could continue to grow until malloc fails?
>>
>> If we want a limit, why don't we take into account the system resources?
>>
>> If we want a constant limit, why don't we put it on threadimpl.h, explain why this value in a comment
>> and document it in thread(2)?
>>
>> Why bother to call malloc before testing if size exeeds that limit???
>
> Given the name of the function (`_threadmalloc`), I'd guess that this isn't
> intended for general use, but rather, for the internal consumption of the
> thread library, where indeed such a large allocation would likely be an
> error (bear in mind this code was developed on 32-bit machines with RAMs
> measured in Megabytes, not Gigabytes).

No, it's used also when creating a channel and setting a thread's
stack size.

adr

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