docs(megbrain): add notes for fastrun

GitOrigin-RevId: b59f7f205d98e127c6dcaaaedfab556cdf2dba21

src/rdnn/impl/algo_chooser.cpp

@@ -565,6 +565,7 @@ typename AlgoChooser<Opr>::ImplExecutionPolicy AlgoChooser<Opr>::AlgoChooserHelp
         choose_by_profile(
                 const ExecutionStrategy& selected_strategy, bool enable_update) const {
     MIDOUT_B(Opr, midout_iv(MGB_HASH_STR("choose_by_profile")))
+    // no_profiling_on_shape_change is usually false, no interface to change it easily
     if (m_desc.no_profiling_on_shape_change) {
         auto policy = m_dnn_opr->execution_policy();
         if (policy.algo.valid()) {
@@ -579,6 +580,8 @@ typename AlgoChooser<Opr>::ImplExecutionPolicy AlgoChooser<Opr>::AlgoChooserHelp
         }
     }
 
+    // if update enabled, do profiling and update cache
+    // enable_update = false only when using HEURISRIC_PROFILE strategy
     typename AlgoChooser<Opr>::ImplExecutionPolicy tmp_policy;
     bool retrive_from_cache = true;
     bool allow_log = false;
@@ -604,6 +607,8 @@ typename AlgoChooser<Opr>::ImplExecutionPolicy AlgoChooser<Opr>::AlgoChooserHelp
         });
     }
 
+    // try to retrive algorithm from fastrun cache, this time it's guaranteed to get
+    // result, retrive_from_cache = true, allow_log = true
     typename AlgoChooser<Opr>::ImplExecutionPolicy policy;
     construct_execution_policy(selected_strategy, policy);
     return policy;
@@ -623,13 +628,16 @@ AlgoChooser<Opr>::AlgoChooserHelper::get_profile_result_from_cache(
             m_incache_layouts.data(), m_incache_layouts.size(), &origin_param,
             sizeof(origin_param)};
     auto&& rst = cache.get(cache_key);
+    // failed to find a cache entry, return
     if (!rst.valid())
         return {{}, rst};
 
+    // found a cache entry(it's a vector of Result), but it's empty
     auto&& prof = rst.val();
     if (prof.empty())
         return {{}, rst};
 
+    // found non-empty cache result, filter it by workspace limit and attribute
     size_t workspace_limit =
             m_desc.get_workspace_limit(m_cn, m_execution_policy.workspace_limit);
     auto target_attr = extract_algo_attribute(selected_strategy);
@@ -644,6 +652,8 @@ AlgoChooser<Opr>::AlgoChooserHelper::get_profile_result_from_cache(
         if (contain_attr_all_positive) {
             if (!contain_attr_any_negative) {
                 if (i.workspace <= workspace_limit) {
+                    // found a well-suited algothrim with good workspace limit and
+                    // correct attribute
                     Algorithm::Info::Desc algo_desc = deserialize_read_pod(i.algo);
                     return {algo_desc, rst};
                 }
@@ -654,9 +664,11 @@ AlgoChooser<Opr>::AlgoChooserHelper::get_profile_result_from_cache(
         }
     }
 
+    // failed to find an algorithm that satisfies the actual workspace limit
     if (skip_by_workspace)
         return {};
 
+    // failed to find an algorithm that satisfies the actual attribute
     std::string layouts_str = AlgoChooser::format_fixlayouts(m_fastrun_layouts);
     if (skip_by_negative) {
         mgb_log_error(
@@ -685,9 +697,12 @@ void AlgoChooser<Opr>::AlgoChooserHelper::construct_execution_policy(
         typename AlgoChooser<Opr>::ImplExecutionPolicy& policy, bool retrive_from_cache,
         bool allow_log) const {
     MIDOUT_B(Opr, midout_iv(MGB_HASH_STR("construct_execution_policy")))
+    // policy.algo is always invalid when called from choose_by_profile
+    // policy.algo will be valid when called from profile
     if (!policy.algo.valid()) {
         if (retrive_from_cache) {
             policy.algo = get_profile_result_from_cache(selected_strategy).first;
+            // nothing is found even with profiling
             if (!policy.algo.valid()) {
                 if (allow_log) {
                     auto target_attr = extract_algo_attribute(selected_strategy);
@@ -710,6 +725,8 @@ void AlgoChooser<Opr>::AlgoChooserHelper::construct_execution_policy(
                 return;
             }
         } else {
+            // retrive_from_cache = false happens when using algo choose hook in
+            // megbrain graph return heuristic algorithm in this case
             auto workspace_limit = m_desc.get_workspace_limit(
                     m_cn, m_execution_policy.workspace_limit);
 
@@ -727,11 +744,13 @@ void AlgoChooser<Opr>::AlgoChooserHelper::construct_execution_policy(
         }
     }
 
+    // construct current algorithm
     Algorithm* algo = m_dnn_opr->get_algorithm_from_desc(policy.algo);
     mgb_assert(algo, "Unknown algo description");
     std::vector<Algorithm::SearchItem>&& sub_items =
             algo->get_subopr_list(to_layout_array<Opr>(m_fastrun_layouts), m_dnn_opr);
 
+    // construct sub oprs' algorithm
     FOREACH_OPR_TYPE_DISPATCH(sub_items, {
         auto&& megdnn_opr = opr::intl::create_megdnn_opr<_Opr>(m_cn);
         megdnn_opr->param() =
@@ -790,6 +809,8 @@ std::vector<typename AlgoChooser<Opr>::ImplAlgo> AlgoChooser<
     auto heu = choose_by_heuristic(m_execution_policy.strategy);
     auto&& ret = APPLY(m_dnn_opr->get_all_algorithms_info(args...), m_fastrun_layouts);
     bool found = false;
+    // make heuristic algorithm always the first in all candidate alrogrithms
+    // so profiling step will always run heuristic algorithm first
     for (size_t i = 0; i < ret.size(); ++i) {
         if (ret[i].desc == heu.algo) {
             found = true;
@@ -798,6 +819,7 @@ std::vector<typename AlgoChooser<Opr>::ImplAlgo> AlgoChooser<
         }
     }
 
+    // make sure heuristic algorithm is valid
     Algorithm* palgo = m_dnn_opr->get_algorithm_from_desc(heu.algo);
     mgb_assert(palgo, "Unknown algo description");
     mgb_assert(
@@ -813,6 +835,7 @@ template <typename Opr>
 Maybe<AlgoChooserProfileCache::ResultEntry> AlgoChooser<Opr>::AlgoChooserHelper::
         profile_single_algo(const ImplExecutionPolicy& policy, double& timeout) const {
     MIDOUT_B(Opr, midout_iv(MGB_HASH_STR("profile_single_algo")))
+    // fill TimedProfiler<Opr>::param and run actual timed profiler
     typename TimedProfiler<Opr>::Param param;
     // force check copy size <= dest len-1 from gcc8 for safe
     param.execution_policy =
@@ -867,7 +890,11 @@ template <typename Opr>
 void AlgoChooser<Opr>::AlgoChooserHelper::profile(
         const ExecutionStrategy& selected_strategy) const {
     MIDOUT_B(Opr, midout_iv(MGB_HASH_STR("profile")))
+    // some sub oprs have beed profiled before
+    // sub oprs won't be checked at the beginning of choose_by_profile
     auto&& rst = get_profile_result_from_cache(selected_strategy);
+    // rst.first.valid means there exists valid algorithms for current opr, just return
+    // otherwise need to profile
     if (rst.first.valid())
         return;
     AlgoChooserProfileCache::Result prof_rst;
@@ -957,6 +984,7 @@ void AlgoChooser<Opr>::AlgoChooserHelper::profile(
             Algorithm::attribute_str(target_attr.second).c_str(), workspace_limit);
     mgb_assert(!prof_rst.empty(), "%s", msg.c_str());
 
+    // append some previous profiled results
     if (rst.second.valid())
         prof_rst.insert(
                 prof_rst.end(), rst.second.val().begin(), rst.second.val().end());