The following changes since commit afc9fcde55296b83f659de9da3cdf044812a6eeb:

Merge remote-tracking branch 'remotes/mst/tags/for_upstream' into staging (2021-10-20 06:10:51 -0700)

for you to fetch changes up to 4b2b3d2653f255ef4259a7689af1956536565901:

coroutine: resize pool periodically instead of limiting size (2021-10-21 18:40:07 +0100)

Performance optimization when guest applications submit a lot of parallel I/O.

This has also been found to improve clang SafeStack performance.

Stefan Hajnoczi (1):

coroutine: resize pool periodically instead of limiting size

include/qemu/coroutine-pool-timer.h | 36 ++++++++++++++++

include/qemu/coroutine.h | 7 ++++

iothread.c | 6 +++

util/coroutine-pool-timer.c | 35 ++++++++++++++++

util/main-loop.c | 5 +++

util/qemu-coroutine.c | 64 ++++++++++++++++-------------

util/meson.build | 1 +

7 files changed, 125 insertions(+), 29 deletions(-)

create mode 100644 include/qemu/coroutine-pool-timer.h

create mode 100644 util/coroutine-pool-timer.c

2.31.1

It was reported that enabling SafeStack reduces IOPS significantly

(>25%) with the following fio benchmark on virtio-blk using a NVMe host

block device:

# fio --rw=randrw --bs=4k --iodepth=64 --runtime=1m --direct=1 \

    --filename=/dev/vdb --name=job1 --ioengine=libaio --thread \

    --group_reporting --numjobs=16 --time_based \

--output=/tmp/fio_result

Serge Guelton and I found that SafeStack is not really at fault, it just

increases the cost of coroutine creation. This fio workload exhausts the

coroutine pool and coroutine creation becomes a bottleneck. Previous

work by Honghao Wang also pointed to excessive coroutine creation.

Creating new coroutines is expensive due to allocating new stacks with

mmap(2) and mprotect(2). Currently there are thread-local and global

pools that recycle old Coroutine objects and their stacks but the

hardcoded size limit of 64 for thread-local pools and 128 for the global

pool is insufficient for the fio benchmark shown above.

This patch changes the coroutine pool algorithm to a simple thread-local

pool without a maximum size limit. Threads periodically shrink the pool

down to a size sufficient for the maximum observed number of coroutines.

The global pool is removed by this patch. It can help to hide the fact

that local pools are easily exhausted, but it's doesn't fix the root

cause. I don't think there is a need for a global pool because QEMU's

threads are long-lived, so let's keep things simple.

Performance of the above fio benchmark is as follows:

Before After

IOPS 60k 97k

Memory usage varies over time as needed by the workload:

VSZ (KB) RSS (KB)

Before fio 4705248 843128

During fio 5747668 (+ ~100 MB) 849280

After fio 4694996 (- ~100 MB) 845184

This confirms that coroutines are indeed being freed when no longer

needed.

Thanks to Serge Guelton for working on identifying the bottleneck with

me!

Reported-by: Tingting Mao <timao@redhat.com>

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

Message-id: 20210913153524.1190696-1-stefanha@redhat.com

Cc: Serge Guelton <sguelton@redhat.com>

Cc: Honghao Wang <wanghonghao@bytedance.com>

Cc: Paolo Bonzini <pbonzini@redhat.com>

Cc: Daniele Buono <dbuono@linux.vnet.ibm.com>

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

[Moved atexit notifier to coroutine_delete() after GitLab CI reported a

memory leak in tests/unit/test-aio-multithread because the Coroutine

object was created in the main thread but runs in an IOThread (where

it's also deleted).

--Stefan]

---

include/qemu/coroutine-pool-timer.h | 36 ++++++++++++++++

include/qemu/coroutine.h | 7 ++++

iothread.c | 6 +++

util/coroutine-pool-timer.c | 35 ++++++++++++++++

util/main-loop.c | 5 +++

util/qemu-coroutine.c | 64 ++++++++++++++++-------------

util/meson.build | 1 +

7 files changed, 125 insertions(+), 29 deletions(-)

create mode 100644 include/qemu/coroutine-pool-timer.h

create mode 100644 util/coroutine-pool-timer.c

diff --git a/include/qemu/coroutine-pool-timer.h b/include/qemu/coroutine-pool-timer.h

new file mode 100644

index XXXXXXX..XXXXXXX

--- /dev/null

+++ b/include/qemu/coroutine-pool-timer.h

@@ -XXX,XX +XXX,XX @@

+/*

+ * QEMU coroutine pool timer

+ *

+ * Copyright (c) 2021 Red Hat, Inc.

+ *

+ * SPDX-License-Identifier: LGPL-2.1-or-later

+ *

+ * This work is licensed under the terms of the GNU LGPL, version 2.1 or later.

+ * See the COPYING.LIB file in the top-level directory.

+ *

+ */

+#ifndef COROUTINE_POOL_TIMER_H

+#define COROUTINE_POOL_TIMER_H

+#include "qemu/osdep.h"

+#include "block/aio.h"

+/**

+ * A timer that periodically resizes this thread's coroutine pool, freeing

+ * memory if there are too many unused coroutines.

+ *

+ * Threads that make heavy use of coroutines should use this. Failure to resize

+ * the coroutine pool can lead to large amounts of memory sitting idle and

+ * never being used after the first time.

+ */

+typedef struct {

+ QEMUTimer *timer;

+} CoroutinePoolTimer;

+

+/* Call this before the thread runs the AioContext */

+void coroutine_pool_timer_init(CoroutinePoolTimer *pt, AioContext *ctx);

+

+/* Call this before the AioContext from the init function is destroyed */

+void coroutine_pool_timer_cleanup(CoroutinePoolTimer *pt);

+

+#endif /* COROUTINE_POOL_TIMER_H */

diff --git a/include/qemu/coroutine.h b/include/qemu/coroutine.h

index XXXXXXX..XXXXXXX 100644

--- a/include/qemu/coroutine.h

+++ b/include/qemu/coroutine.h

@@ -XXX,XX +XXX,XX @@ bool qemu_in_coroutine(void);

*/

bool qemu_coroutine_entered(Coroutine *co);

+/**

+ * Optionally call this function periodically to shrink the thread-local pool

+ * down. Spiky workloads can create many coroutines and then never reach that

+ * level again. Shrinking the pool reclaims memory in this case.

+ */

+void qemu_coroutine_pool_periodic_resize(void);

+

/**

* Provides a mutex that can be used to synchronise coroutines

*/

diff --git a/iothread.c b/iothread.c

index XXXXXXX..XXXXXXX 100644

--- a/iothread.c

+++ b/iothread.c

@@ -XXX,XX +XXX,XX @@

#include "qemu/error-report.h"

#include "qemu/rcu.h"

#include "qemu/main-loop.h"

+#include "qemu/coroutine-pool-timer.h"

typedef ObjectClass IOThreadClass;

@@ -XXX,XX +XXX,XX @@ DECLARE_CLASS_CHECKERS(IOThreadClass, IOTHREAD,

static void *iothread_run(void *opaque)

{

IOThread *iothread = opaque;

+ CoroutinePoolTimer co_pool_timer;

rcu_register_thread();

/*

@@ -XXX,XX +XXX,XX @@ static void *iothread_run(void *opaque)

iothread->thread_id = qemu_get_thread_id();

qemu_sem_post(&iothread->init_done_sem);

+ coroutine_pool_timer_init(&co_pool_timer, iothread->ctx);

+

while (iothread->running) {

/*

* Note: from functional-wise the g_main_loop_run() below can

@@ -XXX,XX +XXX,XX @@ static void *iothread_run(void *opaque)

}

}

+ coroutine_pool_timer_cleanup(&co_pool_timer);

+

g_main_context_pop_thread_default(iothread->worker_context);

rcu_unregister_thread();

return NULL;

diff --git a/util/coroutine-pool-timer.c b/util/coroutine-pool-timer.c

new file mode 100644

index XXXXXXX..XXXXXXX

--- /dev/null

+++ b/util/coroutine-pool-timer.c

@@ -XXX,XX +XXX,XX @@

+/*

+ * QEMU coroutine pool timer

+ *

+ * Copyright (c) 2021 Red Hat, Inc.

+ *

+ * SPDX-License-Identifier: LGPL-2.1-or-later

+ *

+ * This work is licensed under the terms of the GNU LGPL, version 2.1 or later.

+ * See the COPYING.LIB file in the top-level directory.

+ *

+ */

+#include "qemu/coroutine-pool-timer.h"

+

+static void coroutine_pool_timer_cb(void *opaque)

+{

+ CoroutinePoolTimer *pt = opaque;

+ int64_t expiry_time_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) +

+ 15 * NANOSECONDS_PER_SECOND;

+

+ qemu_coroutine_pool_periodic_resize();

+ timer_mod(pt->timer, expiry_time_ns);

+}

+

+void coroutine_pool_timer_init(CoroutinePoolTimer *pt, AioContext *ctx)

+{

+ pt->timer = aio_timer_new(ctx, QEMU_CLOCK_REALTIME, SCALE_NS,

+ coroutine_pool_timer_cb, pt);

+ coroutine_pool_timer_cb(pt);

+}

+

+void coroutine_pool_timer_cleanup(CoroutinePoolTimer *pt)

+{

+ timer_free(pt->timer);

+ pt->timer = NULL;

+}

diff --git a/util/main-loop.c b/util/main-loop.c

index XXXXXXX..XXXXXXX 100644

--- a/util/main-loop.c

+++ b/util/main-loop.c

@@ -XXX,XX +XXX,XX @@

#include "qemu/error-report.h"

#include "qemu/queue.h"

#include "qemu/compiler.h"

+#include "qemu/coroutine-pool-timer.h"

#ifndef _WIN32

#include <sys/wait.h>

@@ -XXX,XX +XXX,XX @@ static int qemu_signal_init(Error **errp)

static AioContext *qemu_aio_context;

static QEMUBH *qemu_notify_bh;

+static CoroutinePoolTimer main_loop_co_pool_timer;

static void notify_event_cb(void *opaque)

{

@@ -XXX,XX +XXX,XX @@ int qemu_init_main_loop(Error **errp)

g_source_set_name(src, "io-handler");

g_source_attach(src, NULL);

g_source_unref(src);

+

+ coroutine_pool_timer_init(&main_loop_co_pool_timer, qemu_aio_context);

+

return 0;

}

diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c

index XXXXXXX..XXXXXXX 100644

--- a/util/qemu-coroutine.c

+++ b/util/qemu-coroutine.c

@@ -XXX,XX +XXX,XX @@

#include "block/aio.h"

enum {

- POOL_BATCH_SIZE = 64,

+ /*

+ * qemu_coroutine_pool_periodic_resize() keeps at least this many

+ * coroutines around.

+ */

+ ALLOC_POOL_MIN = 64,

};

+

/** Free list to speed up creation */

-static QSLIST_HEAD(, Coroutine) release_pool = QSLIST_HEAD_INITIALIZER(pool);

-static unsigned int release_pool_size;

static __thread QSLIST_HEAD(, Coroutine) alloc_pool = QSLIST_HEAD_INITIALIZER(pool);

static __thread unsigned int alloc_pool_size;

+static __thread unsigned int num_coroutines;

+static __thread unsigned int max_coroutines_this_slice;

static __thread Notifier coroutine_pool_cleanup_notifier;

static void coroutine_pool_cleanup(Notifier *n, void *value)

@@ -XXX,XX +XXX,XX @@ Coroutine *qemu_coroutine_create(CoroutineEntry *entry, void *opaque)

if (CONFIG_COROUTINE_POOL) {

co = QSLIST_FIRST(&alloc_pool);

- if (!co) {

- if (release_pool_size > POOL_BATCH_SIZE) {

- /* Slow path; a good place to register the destructor, too. */

- if (!coroutine_pool_cleanup_notifier.notify) {

- coroutine_pool_cleanup_notifier.notify = coroutine_pool_cleanup;

- qemu_thread_atexit_add(&coroutine_pool_cleanup_notifier);

- }

-

- /* This is not exact; there could be a little skew between

- * release_pool_size and the actual size of release_pool. But

- * it is just a heuristic, it does not need to be perfect.

- */

- alloc_pool_size = qatomic_xchg(&release_pool_size, 0);

- QSLIST_MOVE_ATOMIC(&alloc_pool, &release_pool);

- co = QSLIST_FIRST(&alloc_pool);

- }

- }

if (co) {

QSLIST_REMOVE_HEAD(&alloc_pool, pool_next);

alloc_pool_size--;

}

+

+ num_coroutines++;

+ if (num_coroutines > max_coroutines_this_slice) {

+ max_coroutines_this_slice = num_coroutines;

+ }

}

if (!co) {

@@ -XXX,XX +XXX,XX @@ static void coroutine_delete(Coroutine *co)

co->caller = NULL;

if (CONFIG_COROUTINE_POOL) {

- if (release_pool_size < POOL_BATCH_SIZE * 2) {

- QSLIST_INSERT_HEAD_ATOMIC(&release_pool, co, pool_next);

- qatomic_inc(&release_pool_size);

- return;

- }

- if (alloc_pool_size < POOL_BATCH_SIZE) {

- QSLIST_INSERT_HEAD(&alloc_pool, co, pool_next);

- alloc_pool_size++;

- return;

+ if (!coroutine_pool_cleanup_notifier.notify) {

+ coroutine_pool_cleanup_notifier.notify = coroutine_pool_cleanup;

+ qemu_thread_atexit_add(&coroutine_pool_cleanup_notifier);

}

+

+ num_coroutines--;

+ QSLIST_INSERT_HEAD(&alloc_pool, co, pool_next);

+ alloc_pool_size++;

+ return;

}

qemu_coroutine_delete(co);

}

+void qemu_coroutine_pool_periodic_resize(void)

+{

+ unsigned pool_size_target =

+ MAX(ALLOC_POOL_MIN, max_coroutines_this_slice) - num_coroutines;

+ max_coroutines_this_slice = num_coroutines;

+

+ while (alloc_pool_size > pool_size_target) {

+ Coroutine *co = QSLIST_FIRST(&alloc_pool);

+ QSLIST_REMOVE_HEAD(&alloc_pool, pool_next);

+ qemu_coroutine_delete(co);

+ alloc_pool_size--;

+ }

+}

+

void qemu_aio_coroutine_enter(AioContext *ctx, Coroutine *co)

{

QSIMPLEQ_HEAD(, Coroutine) pending = QSIMPLEQ_HEAD_INITIALIZER(pending);

diff --git a/util/meson.build b/util/meson.build

index XXXXXXX..XXXXXXX 100644

--- a/util/meson.build

+++ b/util/meson.build

@@ -XXX,XX +XXX,XX @@ if have_block

util_ss.add(files('buffer.c'))

util_ss.add(files('bufferiszero.c'))

util_ss.add(files('coroutine-@0@.c'.format(config_host['CONFIG_COROUTINE_BACKEND'])))

+ util_ss.add(files('coroutine-pool-timer.c'))

util_ss.add(files('hbitmap.c'))

util_ss.add(files('hexdump.c'))

util_ss.add(files('iova-tree.c'))

2.31.1