On Wed, Apr 30, 2014 at 11:10 AM, Kees k <keeskwekkeb...@gmail.com> wrote:
> Hello all,
>
> I am writing a piece of code to interface with a modbus library
> (libmodbus),which should be really fast. One bottleneck is that I have to
> convert a Local<Array> to uint8_t array:
>
>>> uint8_t req[req_length];
>>> Local<Array> req_arr = Local<Array>::Cast(args[1]);
>>> struct timespec tstart, tstop;
>>> clock_gettime(CLOCK_MONOTONIC, &tstart);
>>> for (int i = 0; i < req_length; i++) {
>>> req[i] = req_arr->Get(i)->Uint32Value();
>>> }
>>> clock_gettime(CLOCK_MONOTONIC, &tstop);
>
> According to benchmarks this piece of code takes about 1 ms with a
> req_length of 80 on a Beaglebone Black (1 MHz), which really surprises me.
> After some digging I found that the conversion 'Uint32Value()' consumes most
> of the time. After leaving out this conversion the benchmark results in 100
> usec.
>
> Does anyone have an idea to improve the performance of this simple
> 'Uint32Value' conversion?
Maybe try switching to
v8::Object::SetIndexedPropertiesToExternalArrayData() with type
kExternalUint32Array. That lets you construct a typed array without
going through (much of) the typed array machinery.
void Create(const FunctionCallbackInfo<Value>& args) {
HandleScope handle_scope(args.GetIsolate());
Local<Object> obj = args[0]->ToObject();
uint32_t nelts = args[1]->Uint32Value();
uint32_t* elts = new uint32_t[nelts];
obj->SetIndexedPropertiesToExternalArrayData(elts,
kExternalUint32Array, nelts);
}
void Process(const FunctionCallbackInfo<Value>& args) {
HandleScope handle_scope(args.GetIsolate());
Local<Object> obj = args[0]->ToObject();
void* data = obj->GetIndexedPropertiesExternalArrayData();
if (data == NULL) return;
uint32_t* elts = static_cast<uint32_t*>(data);
uint32_t nelts = obj->GetIndexedPropertiesExternalArrayDataLength();
// Process the data.
}
Note that you need to take care of releasing the memory again when
you're done with it. Either do that manually or turn the object into
a weak persistent handle and release the memory in your weak callback.
You can also use the typed array API in v8.h but that's only available
in node.js master. (Writing that I realized I used the V8 3.25 API in
the example above; the gist of it is the same in v0.10, however.)
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