Re: [PATCH v4 2/8] grep: skip pthreads overhead when using one thread

> On Thu, Jun 1, 2017 at 11:20 PM, Stefan Beller <sbeller@google.com> wrote:
>> On Thu, Jun 1, 2017 at 11:20 AM, Ævar Arnfjörð Bjarmason
>> <avarab@gmail.com> wrote:
>>
>>> +       if (num_threads == 1)
>>> +               num_threads = 0;
>>
>> I would think that it is easier to maintain the code when keep the 1
>> hard coded, and apply the following diff instead. If we encounter
>> a 0 later on, it is not clear what the original user input was.
>> (Did the user ask for 0 as a proxy for GREP_NUM_THREADS_DEFAULT ?
>> do they care about the number of threads?)
>> It is less complexity in the decision logic here.
>>
>> --8<-- (white space broken)
>> diff --git a/builtin/grep.c b/builtin/grep.c
>> index c6c26e9b9e..6ad9b3da20 100644
>> --- a/builtin/grep.c
>> +++ b/builtin/grep.c
>> @@ -1231,7 +1231,7 @@ int cmd_grep(int argc, const char **argv, const
>> char *prefix)
>>  #endif
>>
>>  #ifndef NO_PTHREADS
>> -       if (num_threads) {
>> +       if (num_threads > 1) {
>>                 if (!(opt.name_only || opt.unmatch_name_only || opt.count)
>>                     && (opt.pre_context || opt.post_context ||
>>                         opt.file_break || opt.funcbody))
>> @@ -1295,7 +1295,7 @@ int cmd_grep(int argc, const char **argv, const
>> char *prefix)
>>                 hit = grep_objects(&opt, &pathspec, &list);
>>         }
>>
>> -       if (num_threads)
>> +       if (num_threads > 1)
>>                 hit |= wait_all();
>>         if (hit && show_in_pager)
>>                 run_pager(&opt, prefix);
>> --8<--
>
> If I've understood you correctly (what I applied on top of this based
> on the above at the end of the mail) this segfaults because now we
> won't compile the pattern.
>
> I agree that this logic is a bit tricky, but it must be considered
> with the "!num_threads" logic in preceding "grep: don't redundantly
> compile throwaway patterns under threading" patch.
>
> I.e. we already have num_threads being assigned to 0 under
> NO_PTHREADS, doing the same for the PTHREADS codepath seemed like the
> simplest solution to me.
>
> diff --git a/builtin/grep.c b/builtin/grep.c
> index a0a3922f92..6d16df2526 100644
> --- a/builtin/grep.c
> +++ b/builtin/grep.c
> @@ -1238,25 +1238,23 @@ int cmd_grep(int argc, const char **argv,
> const char *prefix)
>                 num_threads = GREP_NUM_THREADS_DEFAULT;
>         else if (num_threads < 0)
>                 die(_("invalid number of threads specified (%d)"), num_threads);
> -       if (num_threads == 1)
> -               num_threads = 0;
>  #else
>         if (num_threads)
>                 warning(_("no threads support, ignoring --threads"));
> -       num_threads = 0;
> +       num_threads = 0; /* If we have no threads... */
>  #endif
>
> -       if (!num_threads)
> +       if (num_threads) /* Or if we've decided not to use them... */

>                 /*
>                  * The compiled patterns on the main path are only
>                  * used when not using threading. Otherwise
>                  * start_threads() below calls compile_grep_patterns()
>                  * for each thread.
>                  */
> -               compile_grep_patterns(&opt);
> +               compile_grep_patterns(&opt); /* We'll compile the
> pattern here */
>  #ifndef NO_PTHREADS
> -       if (num_threads) {
> +       if (num_threads > 1) {
>                 if (!(opt.name_only || opt.unmatch_name_only || opt.count)
>                     && (opt.pre_context || opt.post_context ||
>                         opt.file_break || opt.funcbody))
> @@ -1320,7 +1318,7 @@ int cmd_grep(int argc, const char **argv, const
> char *prefix)
>                 hit = grep_objects(&opt, &pathspec, &list);
>         }
>
> -       if (num_threads)
> +       if (num_threads > 1)
>                 hit |= wait_all();
>         if (hit && show_in_pager)
>                 run_pager(&opt, prefix);