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rocksdb/db/write_thread.cc

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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under the BSD-style license found in the
// LICENSE file in the root directory of this source tree. An additional grant
// of patent rights can be found in the PATENTS file in the same directory.
#include "db/write_thread.h"
#include <chrono>
#include <limits>
#include <thread>
#include "db/column_family.h"
#include "port/port.h"
#include "util/sync_point.h"
namespace rocksdb {
WriteThread::WriteThread(uint64_t max_yield_usec, uint64_t slow_yield_usec)
: max_yield_usec_(max_yield_usec),
slow_yield_usec_(slow_yield_usec),
newest_writer_(nullptr) {}
uint8_t WriteThread::BlockingAwaitState(Writer* w, uint8_t goal_mask) {
// We're going to block. Lazily create the mutex. We guarantee
// propagation of this construction to the waker via the
// STATE_LOCKED_WAITING state. The waker won't try to touch the mutex
// or the condvar unless they CAS away the STATE_LOCKED_WAITING that
// we install below.
w->CreateMutex();
auto state = w->state.load(std::memory_order_acquire);
assert(state != STATE_LOCKED_WAITING);
if ((state & goal_mask) == 0 &&
w->state.compare_exchange_strong(state, STATE_LOCKED_WAITING)) {
// we have permission (and an obligation) to use StateMutex
std::unique_lock<std::mutex> guard(w->StateMutex());
w->StateCV().wait(guard, [w] {
return w->state.load(std::memory_order_relaxed) != STATE_LOCKED_WAITING;
});
state = w->state.load(std::memory_order_relaxed);
}
// else tricky. Goal is met or CAS failed. In the latter case the waker
// must have changed the state, and compare_exchange_strong has updated
// our local variable with the new one. At the moment WriteThread never
// waits for a transition across intermediate states, so we know that
// since a state change has occurred the goal must have been met.
assert((state & goal_mask) != 0);
return state;
}
uint8_t WriteThread::AwaitState(Writer* w, uint8_t goal_mask,
AdaptationContext* ctx) {
uint8_t state;
// On a modern Xeon each loop takes about 7 nanoseconds (most of which
// is the effect of the pause instruction), so 200 iterations is a bit
// more than a microsecond. This is long enough that waits longer than
// this can amortize the cost of accessing the clock and yielding.
for (uint32_t tries = 0; tries < 200; ++tries) {
state = w->state.load(std::memory_order_acquire);
if ((state & goal_mask) != 0) {
return state;
}
port::AsmVolatilePause();
}
// If we're only going to end up waiting a short period of time,
// it can be a lot more efficient to call std::this_thread::yield()
// in a loop than to block in StateMutex(). For reference, on my 4.0
// SELinux test server with support for syscall auditing enabled, the
// minimum latency between FUTEX_WAKE to returning from FUTEX_WAIT is
// 2.7 usec, and the average is more like 10 usec. That can be a big
// drag on RockDB's single-writer design. Of course, spinning is a
// bad idea if other threads are waiting to run or if we're going to
// wait for a long time. How do we decide?
//
// We break waiting into 3 categories: short-uncontended,
// short-contended, and long. If we had an oracle, then we would always
// spin for short-uncontended, always block for long, and our choice for
// short-contended might depend on whether we were trying to optimize
// RocksDB throughput or avoid being greedy with system resources.
//
// Bucketing into short or long is easy by measuring elapsed time.
// Differentiating short-uncontended from short-contended is a bit
// trickier, but not too bad. We could look for involuntary context
// switches using getrusage(RUSAGE_THREAD, ..), but it's less work
// (portability code and CPU) to just look for yield calls that take
// longer than we expect. sched_yield() doesn't actually result in any
// context switch overhead if there are no other runnable processes
// on the current core, in which case it usually takes less than
// a microsecond.
//
// There are two primary tunables here: the threshold between "short"
// and "long" waits, and the threshold at which we suspect that a yield
// is slow enough to indicate we should probably block. If these
// thresholds are chosen well then CPU-bound workloads that don't
// have more threads than cores will experience few context switches
// (voluntary or involuntary), and the total number of context switches
// (voluntary and involuntary) will not be dramatically larger (maybe
// 2x) than the number of voluntary context switches that occur when
// --max_yield_wait_micros=0.
//
// There's another constant, which is the number of slow yields we will
// tolerate before reversing our previous decision. Solitary slow
// yields are pretty common (low-priority small jobs ready to run),
// so this should be at least 2. We set this conservatively to 3 so
// that we can also immediately schedule a ctx adaptation, rather than
// waiting for the next update_ctx.
const size_t kMaxSlowYieldsWhileSpinning = 3;
bool update_ctx = false;
bool would_spin_again = false;
if (max_yield_usec_ > 0) {
update_ctx = Random::GetTLSInstance()->OneIn(256);
if (update_ctx || ctx->value.load(std::memory_order_relaxed) >= 0) {
// we're updating the adaptation statistics, or spinning has >
// 50% chance of being shorter than max_yield_usec_ and causing no
// involuntary context switches
auto spin_begin = std::chrono::steady_clock::now();
// this variable doesn't include the final yield (if any) that
// causes the goal to be met
size_t slow_yield_count = 0;
auto iter_begin = spin_begin;
while ((iter_begin - spin_begin) <=
std::chrono::microseconds(max_yield_usec_)) {
std::this_thread::yield();
state = w->state.load(std::memory_order_acquire);
if ((state & goal_mask) != 0) {
// success
would_spin_again = true;
break;
}
auto now = std::chrono::steady_clock::now();
if (now == iter_begin ||
now - iter_begin >= std::chrono::microseconds(slow_yield_usec_)) {
// conservatively count it as a slow yield if our clock isn't
// accurate enough to measure the yield duration
++slow_yield_count;
if (slow_yield_count >= kMaxSlowYieldsWhileSpinning) {
// Not just one ivcsw, but several. Immediately update ctx
// and fall back to blocking
update_ctx = true;
break;
}
}
iter_begin = now;
}
}
}
if ((state & goal_mask) == 0) {
state = BlockingAwaitState(w, goal_mask);
}
if (update_ctx) {
auto v = ctx->value.load(std::memory_order_relaxed);
// fixed point exponential decay with decay constant 1/1024, with +1
// and -1 scaled to avoid overflow for int32_t
v = v + (v / 1024) + (would_spin_again ? 1 : -1) * 16384;
ctx->value.store(v, std::memory_order_relaxed);
}
assert((state & goal_mask) != 0);
return state;
}
void WriteThread::SetState(Writer* w, uint8_t new_state) {
auto state = w->state.load(std::memory_order_acquire);
if (state == STATE_LOCKED_WAITING ||
!w->state.compare_exchange_strong(state, new_state)) {
assert(state == STATE_LOCKED_WAITING);
std::lock_guard<std::mutex> guard(w->StateMutex());
assert(w->state.load(std::memory_order_relaxed) != new_state);
w->state.store(new_state, std::memory_order_relaxed);
w->StateCV().notify_one();
}
}
void WriteThread::LinkOne(Writer* w, bool* linked_as_leader) {
assert(w->state == STATE_INIT);
while (true) {
Writer* writers = newest_writer_.load(std::memory_order_relaxed);
w->link_older = writers;
if (newest_writer_.compare_exchange_strong(writers, w)) {
if (writers == nullptr) {
// this isn't part of the WriteThread machinery, but helps with
// debugging and is checked by an assert in WriteImpl
w->state.store(STATE_GROUP_LEADER, std::memory_order_relaxed);
}
*linked_as_leader = (writers == nullptr);
return;
}
}
}
void WriteThread::CreateMissingNewerLinks(Writer* head) {
while (true) {
Writer* next = head->link_older;
if (next == nullptr || next->link_newer != nullptr) {
assert(next == nullptr || next->link_newer == head);
break;
}
next->link_newer = head;
head = next;
}
}
void WriteThread::JoinBatchGroup(Writer* w) {
static AdaptationContext ctx("JoinBatchGroup");
assert(w->batch != nullptr);
bool linked_as_leader;
LinkOne(w, &linked_as_leader);
TEST_SYNC_POINT_CALLBACK("WriteThread::JoinBatchGroup:Wait", w);
if (!linked_as_leader) {
AwaitState(w,
STATE_GROUP_LEADER | STATE_PARALLEL_FOLLOWER | STATE_COMPLETED,
&ctx);
TEST_SYNC_POINT_CALLBACK("WriteThread::JoinBatchGroup:DoneWaiting", w);
}
}
size_t WriteThread::EnterAsBatchGroupLeader(
Writer* leader, WriteThread::Writer** last_writer,
autovector<WriteThread::Writer*>* write_batch_group) {
assert(leader->link_older == nullptr);
assert(leader->batch != nullptr);
size_t size = WriteBatchInternal::ByteSize(leader->batch);
write_batch_group->push_back(leader);
// Allow the group to grow up to a maximum size, but if the
// original write is small, limit the growth so we do not slow
// down the small write too much.
size_t max_size = 1 << 20;
if (size <= (128 << 10)) {
max_size = size + (128 << 10);
}
*last_writer = leader;
Writer* newest_writer = newest_writer_.load(std::memory_order_acquire);
// This is safe regardless of any db mutex status of the caller. Previous
// calls to ExitAsGroupLeader either didn't call CreateMissingNewerLinks
// (they emptied the list and then we added ourself as leader) or had to
// explicitly wake us up (the list was non-empty when we added ourself,
// so we have already received our MarkJoined).
CreateMissingNewerLinks(newest_writer);
// Tricky. Iteration start (leader) is exclusive and finish
// (newest_writer) is inclusive. Iteration goes from old to new.
Writer* w = leader;
while (w != newest_writer) {
w = w->link_newer;
if (w->sync && !leader->sync) {
// Do not include a sync write into a batch handled by a non-sync write.
break;
}
if (w->no_slowdown != leader->no_slowdown) {
// Do not mix writes that are ok with delays with the ones that
// request fail on delays.
break;
}
if (!w->disableWAL && leader->disableWAL) {
// Do not include a write that needs WAL into a batch that has
// WAL disabled.
break;
}
if (w->batch == nullptr) {
// Do not include those writes with nullptr batch. Those are not writes,
// those are something else. They want to be alone
break;
}
if (w->callback != nullptr && !w->callback->AllowWriteBatching()) {
// dont batch writes that don't want to be batched
break;
}
auto batch_size = WriteBatchInternal::ByteSize(w->batch);
if (size + batch_size > max_size) {
// Do not make batch too big
break;
}
size += batch_size;
write_batch_group->push_back(w);
w->in_batch_group = true;
*last_writer = w;
}
return size;
}
void WriteThread::LaunchParallelFollowers(ParallelGroup* pg,
SequenceNumber sequence) {
// EnterAsBatchGroupLeader already created the links from leader to
// newer writers in the group
pg->leader->parallel_group = pg;
Writer* w = pg->leader;
w->sequence = sequence;
while (w != pg->last_writer) {
// Writers that won't write don't get sequence allotment
if (!w->CallbackFailed() && w->ShouldWriteToMemtable()) {
sequence += WriteBatchInternal::Count(w->batch);
}
w = w->link_newer;
w->sequence = sequence;
w->parallel_group = pg;
SetState(w, STATE_PARALLEL_FOLLOWER);
}
}
bool WriteThread::CompleteParallelWorker(Writer* w) {
static AdaptationContext ctx("CompleteParallelWorker");
auto* pg = w->parallel_group;
if (!w->status.ok()) {
std::lock_guard<std::mutex> guard(pg->leader->StateMutex());
pg->status = w->status;
}
auto leader = pg->leader;
auto early_exit_allowed = pg->early_exit_allowed;
if (pg->running.load(std::memory_order_acquire) > 1 && pg->running-- > 1) {
// we're not the last one
AwaitState(w, STATE_COMPLETED, &ctx);
// Caller only needs to perform exit duties if early exit doesn't
// apply and this is the leader. Can't touch pg here. Whoever set
// our state to STATE_COMPLETED copied pg->status to w.status for us.
return w == leader && !(early_exit_allowed && w->status.ok());
}
// else we're the last parallel worker
if (w == leader || (early_exit_allowed && pg->status.ok())) {
// this thread should perform exit duties
w->status = pg->status;
return true;
} else {
// We're the last parallel follower but early commit is not
// applicable. Wake up the leader and then wait for it to exit.
assert(w->state == STATE_PARALLEL_FOLLOWER);
SetState(leader, STATE_COMPLETED);
AwaitState(w, STATE_COMPLETED, &ctx);
return false;
}
}
void WriteThread::EarlyExitParallelGroup(Writer* w) {
auto* pg = w->parallel_group;
assert(w->state == STATE_PARALLEL_FOLLOWER);
assert(pg->status.ok());
ExitAsBatchGroupLeader(pg->leader, pg->last_writer, pg->status);
assert(w->status.ok());
assert(w->state == STATE_COMPLETED);
SetState(pg->leader, STATE_COMPLETED);
}
void WriteThread::ExitAsBatchGroupLeader(Writer* leader, Writer* last_writer,
Status status) {
assert(leader->link_older == nullptr);
Writer* head = newest_writer_.load(std::memory_order_acquire);
if (head != last_writer ||
!newest_writer_.compare_exchange_strong(head, nullptr)) {
// Either w wasn't the head during the load(), or it was the head
// during the load() but somebody else pushed onto the list before
// we did the compare_exchange_strong (causing it to fail). In the
// latter case compare_exchange_strong has the effect of re-reading
// its first param (head). No need to retry a failing CAS, because
// only a departing leader (which we are at the moment) can remove
// nodes from the list.
assert(head != last_writer);
// After walking link_older starting from head (if not already done)
// we will be able to traverse w->link_newer below. This function
// can only be called from an active leader, only a leader can
// clear newest_writer_, we didn't, and only a clear newest_writer_
// could cause the next leader to start their work without a call
// to MarkJoined, so we can definitely conclude that no other leader
// work is going on here (with or without db mutex).
CreateMissingNewerLinks(head);
assert(last_writer->link_newer->link_older == last_writer);
last_writer->link_newer->link_older = nullptr;
// Next leader didn't self-identify, because newest_writer_ wasn't
// nullptr when they enqueued (we were definitely enqueued before them
// and are still in the list). That means leader handoff occurs when
// we call MarkJoined
SetState(last_writer->link_newer, STATE_GROUP_LEADER);
}
// else nobody else was waiting, although there might already be a new
// leader now
while (last_writer != leader) {
last_writer->status = status;
// we need to read link_older before calling SetState, because as soon
// as it is marked committed the other thread's Await may return and
// deallocate the Writer.
auto next = last_writer->link_older;
SetState(last_writer, STATE_COMPLETED);
last_writer = next;
}
}
void WriteThread::EnterUnbatched(Writer* w, InstrumentedMutex* mu) {
static AdaptationContext ctx("EnterUnbatched");
assert(w->batch == nullptr);
bool linked_as_leader;
LinkOne(w, &linked_as_leader);
if (!linked_as_leader) {
mu->Unlock();
TEST_SYNC_POINT("WriteThread::EnterUnbatched:Wait");
AwaitState(w, STATE_GROUP_LEADER, &ctx);
mu->Lock();
}
}
void WriteThread::ExitUnbatched(Writer* w) {
Status dummy_status;
ExitAsBatchGroupLeader(w, w, dummy_status);
}
} // namespace rocksdb