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

[Ben Peart]

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.