Subject: Re: [PATCH v2 2/2] range-diff: add configurable memory limit for cost matrix

From: Junio C Hamano <[email-masked]>
Date: Thu, 28 Aug 2025 14:22:45 -0700

Elijah Newren <[email-masked]> writes:

> <[email-masked]> wrote:
>>
>> From: pcasaretto <[email-masked]>
>> Signed-off-by: Paulo Casaretto <[email-masked]>
>
> The names (and emails) in these should match; I believe the name in
> the From field is set by Gitgitgadget based on your profile settings;
> see https://github.com/settings/profile and set your name there.
>
>>  static void get_correspondences(struct string_list *a, struct string_list *b,
>> -                               int creation_factor)
>> +                               int creation_factor, size_t max_memory)
>>  {
>>         int n = a->nr + b->nr;
>>         int *cost, c, *a2b, *b2a;
>>         int i, j;
>> -
>> -       ALLOC_ARRAY(cost, st_mult(n, n));
>> +       size_t cost_size = st_mult(n, n);
>> +       size_t cost_bytes = st_mult(sizeof(int), cost_size);
>> +       if (cost_bytes >= max_memory) {
>> +               struct strbuf cost_str = STRBUF_INIT;
>> +               struct strbuf max_str = STRBUF_INIT;
>> +               strbuf_humanise_bytes(&cost_str, cost_bytes);
>> +               strbuf_humanise_bytes(&max_str, max_memory);
>> +               die(_("range-diff: unable to compute the range-diff, since it "
>> +                     "exceeds the maximum memory for the cost matrix: %s "
>> +                     "(%"PRIuMAX" bytes) needed, %s (%"PRIuMAX" bytes) available"),
>
> available?  I'm worried the error message will report in users
> checking system memory, claiming they have 14GB available on their
> system, and then reporting a "bug".
>
> Perhaps something like:
>
> +                     "(%"PRIuMAX" bytes) needed, limited to %s
> (%"PRIuMAX" bytes)"),

Sounds like a good idea.

I am not a huge fan of configuration variables that do not have a
command line option.  Assuming that it is not like you'd be doing
overly huge range-diff that would not fit your memory every day,
shouldn't we start this with a command line option without a
configuration variable to gauge how useful it would be for users
with such a need, and then after it proves useful and we identify a
workflow where a user would be passing this option all the time, add
a configuration to allow it always be in effect (with command line
override still available)?