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On Thread Start Callback

Motivation:

This is a community driven feature following issue Github #476.
This feature provides the ability to limit the Speedb threads to run on specific cores in order to gain performance or better control CPU usage.
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Solution:

Speedb introduces a new capability that allows Speedb users to configure a callback function that will run for each Speedb thread that is being spawned.
Using this callback a user could configure the thread affinity the way the user wants.

How to use it?

An example is also available under: examples/on_thread_start_callback_example.cc
// This example will pin all Speedb threads to the first core
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#include <functional>
#include <iostream>
#include <memory>
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#include "rocksdb/db.h"
#include "rocksdb/options.h"
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using namespace ROCKSDB_NAMESPACE;
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#if defined(OS_WIN)
std::string kDBPath = "C:\\Windows\\TEMP\\speedb_thr_affinity";
#else
std::string kDBPath = "/tmp/speedb_thr_affinity";
#endif
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int main() {
// Open the storage
DB* db = nullptr;
Options options;
// create the DB if it's not already present
options.create_if_missing = true;
auto f = [](std::thread::native_handle_type thr) {
// callback to pin all Speedb threads to the first core.
#if defined(OS_WIN)
#include "winbase.h"
SetThreadAffinityMask(thr, 0);
#else
#include "pthread.h"
std::cout << "thread spawned, thread_id: " << thr << std::endl;
cpu_set_t cpuset;
CPU_ZERO(&cpuset);
CPU_SET(0, &cpuset);
pthread_setaffinity_np(thr, sizeof(cpu_set_t), &cpuset);
#endif
};
options.on_thread_start_callback =
std::make_shared<std::function<void(std::thread::native_handle_type)>>(f);
Status s = DB::Open(options, kDBPath, &db);
assert(s.ok());
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// append new entry
std::string key = "key_1";
std::string put_value = "Speedb is awesome!";
s = db->Put(WriteOptions(), key, put_value);
assert(s.ok());
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// retrieve entry
std::string get_value;
s = db->Get(ReadOptions(), key, &get_value);
assert(s.ok());
assert(get_value == put_value);
std::cout << get_value << std::endl;
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// close DB
s = db->Close();
assert(s.ok());
return 0;
}
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