[PATCH] D60609: Use native llvm JSON library for time profiler output

[Anton Afanasyev via Phabricator via llvm-commits]

anton-afanasyev marked an inline comment as done.
anton-afanasyev added inline comments.


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Comment at: llvm/lib/Support/TimeProfiler.cpp:83-86
+    json::Array Events;
 
     // Emit all events for the main flame graph.
     for (const auto &E : Entries) {
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lebedev.ri wrote:
> anton-afanasyev wrote:
> > lebedev.ri wrote:
> > > anton-afanasyev wrote:
> > > > anton-afanasyev wrote:
> > > > > lebedev.ri wrote:
> > > > > > There is no `reserve()` function in `json::Array`, thus you end up doing the memory dance *every* entry. :/
> > > > > > (alloc larger, move, delete previous)
> > > > > > I suspect this accounts for **majority** of the extra cost, although not all of it.
> > > > > Good point to change in json lib.
> > > > I've measured the perf effect of adding `Array.reserve(33000)` (changed `JSON.h` a bit), it gives 3845 msec (vs 3995 msec) for `-time-trace-granularity=0`. Effect is noticeable, but actual implementation need a smart heuristics to determine the entries number to reserve. For the default option (time-trace-granularity = 500 msec) this doesn't matter.
> > > If you don't intend to look into that (should be rather trivial),
> > > can you please at least raise a bug about this, with those perf numbers?
> > What do you mean by bug? Do you mean I should add `Array.reserve(N)` with `N` taken heuristically from different runs for different `time-trace-granularity`? Or would just `Array.reserve(4000)` be enough for default granularity?
> > What do you mean by bug? 
> The lack of `json::array::reserve()`
> 
> >  Do you mean I should add Array.reserve(N) with N taken heuristically from different runs for different time-trace-granularity? Or would just Array.reserve(4000) be enough for default granularity?
> 
> Hm?
> You know how many Events in `json::Array` you will end up with - `Entries.size() + CountAndTotalPerName.size() + 1`.
> You can even assert that in the end.
Also this result means that time taken by json output could be decreased by 40% (`1 - (3845-3634)/(3995-3634)`), but this is actual and significant only for `time-trace-granularity=0`. The effect of array reservation for the default option value is insignificant.


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  https://reviews.llvm.org/D60609/new/

https://reviews.llvm.org/D60609