1b7eea0f52
This patch flips two "switches": 1) It switches admission to be up-front. 2) It changes the admission algorithm. (1) by now all permits are obtained up-front, so this patch just yanks out the restricted reader from all reader stacks and simultaneously switches all `obtain_permit_nowait()` calls to `obtain_permit()`. By doing this admission is now waited on when creating the permit. (2) we switch to an admission algorithm that adds a new aspect to the existing resource availability: the number of used/blocked reads. Namely it only admits new reads if in addition to the necessary amount of resources being available, all currently used readers are blocked. In other words we only admit new reads if all currently admitted reads requires something other than CPU to progress. They are either waiting on I/O, a remote shard, or attention from their consumers (not used currently). We flip these two switches at the same time because up-front admission means cache reads now need to obtain a permit too. For cache reads the optimal concurrency is 1. Anything above that just increases latency (without increasing throughput). So we want to make sure that if a cache reader hits it doesn't get any competition for CPU and it can run to completion. We admit new reads only if the read misses and has to go to disk. Another change made to accommodate this switch is the replacement of the replica side read execution stages which the reader concurrency semaphore as an execution stage. This replacement is needed because with the introduction of up-front admission, reads are not independent of each other any-more. One read executed can influence whether later reads executed will be admitted or not, and execution stages require independent operations to work well. By moving the execution stage into the semaphore, we have an execution stage which is in control of both admission and running the operations in batches, avoiding the bad interaction between the two.
109 lines
4.2 KiB
C++
109 lines
4.2 KiB
C++
/*
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* Licensed to the Apache Software Foundation (ASF) under one
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* or more contributor license agreements. See the NOTICE file
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* distributed with this work for additional information
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* regarding copyright ownership. The ASF licenses this file
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* to you under the Apache License, Version 2.0 (the
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* "License"); you may not use this file except in compliance
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* with the License. You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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/*
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* Modified by ScyllaDB
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* Copyright (C) 2021-present ScyllaDB
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*/
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/*
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* This file is part of Scylla.
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*
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* Scylla is free software: you can redistribute it and/or modify
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* it under the terms of the GNU Affero General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* Scylla is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with Scylla. If not, see <http://www.gnu.org/licenses/>.
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*/
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#include "db/virtual_table.hh"
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#include "db/chained_delegating_reader.hh"
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namespace db {
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void virtual_table::set_cell(row& cr, const bytes& column_name, data_value value) {
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auto ts = api::new_timestamp();
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auto cdef = schema()->get_column_definition(column_name);
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if (!cdef) {
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throw_with_backtrace<std::runtime_error>(format("column not found: {}", column_name));
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}
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if (!value.is_null()) {
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cr.apply(*cdef, atomic_cell::make_live(*cdef->type, ts, value.serialize_nonnull()));
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}
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}
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bool virtual_table::this_shard_owns(const dht::decorated_key& dk) const {
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return dht::shard_of(*_s, dk.token()) == this_shard_id();
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}
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bool virtual_table::contains_key(const dht::partition_range& pr, const dht::decorated_key& dk) const {
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return pr.contains(dk, dht::ring_position_comparator(*_s));
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}
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mutation_source memtable_filling_virtual_table::as_mutation_source() {
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return mutation_source([this] (schema_ptr s,
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reader_permit permit,
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const dht::partition_range& range,
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const query::partition_slice& slice,
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const io_priority_class& pc,
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tracing::trace_state_ptr trace_state,
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streamed_mutation::forwarding fwd,
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mutation_reader::forwarding fwd_mr) {
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struct my_units {
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reader_permit::resource_units units;
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uint64_t memory_used;
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my_units(reader_permit::resource_units&& units) : units(std::move(units)), memory_used(0) {}
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};
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auto units = make_lw_shared<my_units>(permit.consume_memory(0));
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auto populate = [this, mt = make_lw_shared<memtable>(schema()), s, units, range, slice, pc, trace_state, fwd, fwd_mr] (db::timeout_clock::time_point timeout) mutable {
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auto mutation_sink = [units, mt] (mutation m) mutable {
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mt->apply(m);
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units->units.add(units->units.permit().consume_memory(mt->occupancy().used_space() - units->memory_used));
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units->memory_used = mt->occupancy().used_space();
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};
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return execute(mutation_sink, timeout).then([this, mt, s, units, &range, &slice, &pc, &trace_state, &fwd, &fwd_mr] () {
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auto rd = mt->as_data_source().make_reader(s, units->units.permit(), range, slice, pc, trace_state, fwd, fwd_mr);
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if (!_shard_aware) {
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rd = make_filtering_reader(std::move(rd), [this] (const dht::decorated_key& dk) -> bool {
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return this_shard_owns(dk);
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});
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}
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return rd;
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});
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};
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// populate keeps the memtable alive.
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return make_flat_mutation_reader<chained_delegating_reader>(s, std::move(populate), units->units.permit());
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});
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}
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} |