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Change fastlock implementation from naive spinlock to a ticket lock for better fairness and lower peak latency

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This commit is contained in:
John Sully 2019-02-21 00:16:47 -05:00
parent 02e7fe400c
commit 942510f1a5
2 changed files with 53 additions and 17 deletions
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60
src/fastlock.cpp
View File

@ -1,3 +1,32 @@
/*
* Copyright (c) 2019, John Sully <john at eqalpha dot com>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* * Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
* * Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* * Neither the name of Redis nor the names of its contributors may be used
* to endorse or promote products derived from this software without
* specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/
#include "fastlock.h"
#include <unistd.h>
#include <sys/syscall.h>
@ -5,6 +34,13 @@
#include <sched.h>
#include <atomic>
/****************************************************
*
* Implementation of a fair spinlock. To promote fairness we
* use a ticket lock instead of a raw spinlock
*
****************************************************/
static_assert(sizeof(pid_t) <= sizeof(fastlock::m_pidOwner), "fastlock::m_pidOwner not large enough");
static pid_t gettid()
@ -17,22 +53,25 @@ static pid_t gettid()
extern "C" void fastlock_init(struct fastlock *lock)
{
lock->m_lock = 0;
lock->m_active = 0;
lock->m_avail = 0;
lock->m_depth = 0;
}
extern "C" void fastlock_lock(struct fastlock *lock)
{
if (!__sync_bool_compare_and_swap(&lock->m_lock, 0, 1))
{
if (lock->m_pidOwner == gettid())
if ((int)__atomic_load_4(&lock->m_pidOwner, __ATOMIC_ACQUIRE) == gettid())
{
++lock->m_depth;
return;
}
unsigned myticket = __atomic_fetch_add(&lock->m_avail, 1, __ATOMIC_ACQ_REL);
if (__atomic_load_4(&lock->m_active, __ATOMIC_ACQUIRE) != myticket)
{
int cloops = 1;
while (!__sync_bool_compare_and_swap(&lock->m_lock, 0, 1))
while (__atomic_load_4(&lock->m_active, __ATOMIC_ACQUIRE) != myticket)
{
if ((++cloops % 1024*1024) == 0)
sched_yield();
@ -40,7 +79,8 @@ extern "C" void fastlock_lock(struct fastlock *lock)
}
lock->m_depth = 1;
lock->m_pidOwner = gettid();
__atomic_store_4(&lock->m_pidOwner, gettid(), __ATOMIC_RELEASE);
__sync_synchronize();
}
extern "C" void fastlock_unlock(struct fastlock *lock)
@ -50,8 +90,7 @@ extern "C" void fastlock_unlock(struct fastlock *lock)
{
lock->m_pidOwner = -1;
__sync_synchronize();
if (!__sync_bool_compare_and_swap(&lock->m_lock, 1, 0))
*((volatile int*)0) = -1;
__atomic_fetch_add(&lock->m_active, 1, __ATOMIC_ACQ_REL);
}
}
@ -64,10 +103,5 @@ extern "C" void fastlock_free(struct fastlock *lock)
bool fastlock::fOwnLock()
{
if (__sync_bool_compare_and_swap(&m_lock, 0, 1))
{
__sync_bool_compare_and_swap(&m_lock, 1, 0);
return false; // it was never locked
}
return gettid() == m_pidOwner;
}

4
src/fastlock.h
View File

@ -18,7 +18,9 @@ void fastlock_free(struct fastlock *lock);
struct fastlock
{
volatile int m_lock;
volatile unsigned m_active;
volatile unsigned m_avail;
volatile int m_pidOwner;
volatile int m_depth;