Re: [PATCH 4/4] fsmonitor: Delay updating state until after split index is merged

[Ben Peart <peartben@gmail.com>]

On 10/23/2017 5:57 AM, Johannes Schindelin wrote:
> Hi Peff,
> 
> On Fri, 20 Oct 2017, Jeff King wrote:
> 
>> On Fri, Oct 20, 2017 at 03:16:20PM +0200, Johannes Schindelin wrote:
>>
>>>>   void tweak_fsmonitor(struct index_state *istate)
>>>>   {
>>>> +	int i;
>>>> +
>>>> +	if (istate->fsmonitor_dirty) {
>>>> +		/* Mark all entries valid */
>>>> +		trace_printf_key(&trace_fsmonitor, "fsmonitor is enabled; cache is %d", istate->cache_nr);
>>>
>>> Sadly, a call to trace_printf_key() is not really a noop when tracing is
>>> disabled. The call to trace_printf_key() hands off to trace_vprintf_fl(),
>>> which in turn calls prepare_trace_line() which asks trace_want() whether
>>> we need to trace, which finally calls get_trace_fd(). This last function
>>> initializes a trace key if needed, and this entire call stack takes time.
>>
>> It seems like we could pretty easily turn noop traces into a trivial
>> conditional, like:
>>
>> diff --git a/trace.h b/trace.h
>> index 179b249c59..c46b92cbde 100644
>> --- a/trace.h
>> +++ b/trace.h
>> @@ -80,8 +80,11 @@ extern void trace_performance_since(uint64_t start, const char *format, ...);
>>   #define trace_printf(...) \
>>   	trace_printf_key_fl(TRACE_CONTEXT, __LINE__, NULL, __VA_ARGS__)
>>   
>> -#define trace_printf_key(key, ...) \
>> -	trace_printf_key_fl(TRACE_CONTEXT, __LINE__, key, __VA_ARGS__)
>> +#define trace_printf_key(key, ...) do { \
>> +	if (!key->initialized || key->fd) \
>> +		trace_printf_key_fl(TRACE_CONTEXT, __LINE__, key, __VA_ARGS__) \
>> +} while(0)
>> +
>>   
>>   #define trace_argv_printf(argv, ...) \
>>   	trace_argv_printf_fl(TRACE_CONTEXT, __LINE__, argv, __VA_ARGS__)
>>
>>
>> (OK, that's got an OR, but if we are really pinching instructions we
>> could obviously store a single "I've been initialized and am disabled"
>> flag).
> 
> I'd really like that.
> 
>> I don't have an opinion one way or the other on these particular
>> messages, but in general I'd like to see _more_ tracing in Git, not
>> less. I've often traced Git with a debugger or other tools like perf,
>> but there's real value in the author of code annotating high-level
>> logical events.
> 
> Yes, I like that idea, too: to add more tracing.
> 
> Thanks,
> Dscho
> 

Two big thumbs up.

I find turning on tracking very informative as I can better see the flow 
of execution and sometimes am surprised by what I discover. :)

I've often added additional tracing while working on a feature only to 
remove it before submitting the patch because of the performance 
overhead.  Being able to leave that in would be very helpful.