Re: [Qemu-devel] [RFC v3 PATCH 2/5] cpus-common: Cache allocated work items

[Alex Bennée]

Pranith Kumar  writes:

> Using heaptrack, I found that quite a few of our temporary allocations
> are coming from allocating work items. Instead of doing this
> continously, we can cache the allocated items and reuse them instead
> of freeing them.
>
> Stats from an ARM64 guest (boot+shutdown):
>
> heaptrack stats(before):
> allocations:1471317
> leaked allocations: 73824
> temporary allocations:  651293
>
> heaptrack stats(after):
> allocations:1143130
> leaked allocations: 73693
> temporary allocations:  487342
>
> The improvement in speedup is minor and within error margins, however I think 
> the
> patch is still worth. We can also explore atomics instead of taking a lock for
> the work item pool.

When we where doing the original MTTCG work I looked at using GArray for
the work queue, see:

  http://lists.gnu.org/archive/html/qemu-devel/2016-08/msg00367.html

specifically:

  Subject: [PATCH v5 13/13] cpu-exec: replace cpu->queued_work with GArray
  Date: Tue,  2 Aug 2016 18:27:44 +0100
  Message-Id: <1470158864-17651-14-git-send-email-alex.ben...@linaro.org>

which I personally think might yield better results than messing around
with custom allocators and GSlice and the like. You still get the
dynamic sizing of a malloc based array but for operations like insertion
and iterating through the work queue should be cache friendly.

Once the array has (transparently) reached a reasonable size to service
all allocations in the usual servicing period the same memory can be
used over and over again ;-)

My fondness for arrays is informed by comments by Bjarne Stroustrup:

  https://www.youtube.com/watch?v=YQs6IC-vgmo

Obviously this patch would need to be re-worked given how much the code
has changes since it was merged.

>
> Signed-off-by: Pranith Kumar 
> ---
>  cpus-common.c | 75 
> +++
>  1 file changed, 60 insertions(+), 15 deletions(-)
>
> diff --git a/cpus-common.c b/cpus-common.c
> index 59f751ecf9..ccf5f50e4e 100644
> --- a/cpus-common.c
> +++ b/cpus-common.c
> @@ -24,6 +24,7 @@
>  #include "sysemu/cpus.h"
>
>  static QemuMutex qemu_cpu_list_lock;
> +static QemuMutex qemu_wi_pool_lock;
>  static QemuCond exclusive_cond;
>  static QemuCond exclusive_resume;
>  static QemuCond qemu_work_cond;
> @@ -33,6 +34,49 @@ static QemuCond qemu_work_cond;
>   */
>  static int pending_cpus;
>
> +typedef struct qemu_work_item {
> +struct qemu_work_item *next;
> +run_on_cpu_func func;
> +run_on_cpu_data data;
> +bool free, exclusive, done;
> +} qemu_work_item;
> +
> +typedef struct qemu_wi_pool {
> +qemu_work_item *head;
> +int num_items;
> +} qemu_wi_pool;
> +
> +qemu_wi_pool *wi_free_pool;
> +
> +static void qemu_init_workitem_pool(void)
> +{
> +wi_free_pool = g_malloc0(sizeof(qemu_wi_pool));
> +}
> +
> +static void qemu_wi_pool_insert(qemu_work_item *item)
> +{
> +qemu_mutex_lock(_wi_pool_lock);
> +qemu_work_item *curr = atomic_read(_free_pool->head);
> +item->next = curr;
> +wi_free_pool->head = item;
> +qemu_mutex_unlock(_wi_pool_lock);
> +}
> +
> +static qemu_work_item *qemu_wi_pool_remove(void)
> +{
> +qemu_mutex_lock(_wi_pool_lock);
> +qemu_work_item *curr = atomic_read(_free_pool->head);
> +if (curr == NULL) {
> +goto out;
> +}
> +wi_free_pool->head = curr->next;
> +curr->next = NULL;
> +
> + out:
> +qemu_mutex_unlock(_wi_pool_lock);
> +return curr;
> +}
> +
>  void qemu_init_cpu_list(void)
>  {
>  /* This is needed because qemu_init_cpu_list is also called by the
> @@ -43,6 +87,9 @@ void qemu_init_cpu_list(void)
>  qemu_cond_init(_cond);
>  qemu_cond_init(_resume);
>  qemu_cond_init(_work_cond);
> +
> +qemu_init_workitem_pool();
> +qemu_mutex_init(_wi_pool_lock);
>  }
>
>  void cpu_list_lock(void)
> @@ -106,14 +153,7 @@ void cpu_list_remove(CPUState *cpu)
>  qemu_mutex_unlock(_cpu_list_lock);
>  }
>
> -struct qemu_work_item {
> -struct qemu_work_item *next;
> -run_on_cpu_func func;
> -run_on_cpu_data data;
> -bool free, exclusive, done;
> -};
> -
> -static void queue_work_on_cpu(CPUState *cpu, struct qemu_work_item *wi)
> +static void queue_work_on_cpu(CPUState *cpu, qemu_work_item *wi)
>  {
>  qemu_mutex_lock(>work_mutex);
>  if (cpu->queued_work_first == NULL) {
> @@ -132,7 +172,7 @@ static void queue_work_on_cpu(CPUState *cpu, struct 
> qemu_work_item *wi)
>  void do_run_on_cpu(CPUState *cpu, run_on_cpu_func func, run_on_cpu_data data,
> QemuMutex *mutex)
>  {
> -struct qemu_work_item wi;
> +qemu_work_item wi;
>
>  if (qemu_cpu_is_self(cpu)) {
>  func(cpu, data);
> @@ -156,9 +196,11 @@ void do_run_on_cpu(CPUState *cpu, run_on_cpu_func func, 
> run_on_cpu_data data,
>
>  void async_run_on_cpu(CPUState *cpu, run_on_cpu_func func, run_on_cpu_data 

[Qemu-devel] [RFC v3 PATCH 2/5] cpus-common: Cache allocated work items

[Pranith Kumar]

Using heaptrack, I found that quite a few of our temporary allocations
are coming from allocating work items. Instead of doing this
continously, we can cache the allocated items and reuse them instead
of freeing them.

Stats from an ARM64 guest (boot+shutdown):

heaptrack stats(before):
allocations:1471317
leaked allocations: 73824
temporary allocations:  651293

heaptrack stats(after):
allocations:1143130
leaked allocations: 73693
temporary allocations:  487342

The improvement in speedup is minor and within error margins, however I think 
the
patch is still worth. We can also explore atomics instead of taking a lock for
the work item pool.

Signed-off-by: Pranith Kumar 
---
 cpus-common.c | 75 +++
 1 file changed, 60 insertions(+), 15 deletions(-)

diff --git a/cpus-common.c b/cpus-common.c
index 59f751ecf9..ccf5f50e4e 100644
--- a/cpus-common.c
+++ b/cpus-common.c
@@ -24,6 +24,7 @@
 #include "sysemu/cpus.h"
 
 static QemuMutex qemu_cpu_list_lock;
+static QemuMutex qemu_wi_pool_lock;
 static QemuCond exclusive_cond;
 static QemuCond exclusive_resume;
 static QemuCond qemu_work_cond;
@@ -33,6 +34,49 @@ static QemuCond qemu_work_cond;
  */
 static int pending_cpus;
 
+typedef struct qemu_work_item {
+struct qemu_work_item *next;
+run_on_cpu_func func;
+run_on_cpu_data data;
+bool free, exclusive, done;
+} qemu_work_item;
+
+typedef struct qemu_wi_pool {
+qemu_work_item *head;
+int num_items;
+} qemu_wi_pool;
+
+qemu_wi_pool *wi_free_pool;
+
+static void qemu_init_workitem_pool(void)
+{
+wi_free_pool = g_malloc0(sizeof(qemu_wi_pool));
+}
+
+static void qemu_wi_pool_insert(qemu_work_item *item)
+{
+qemu_mutex_lock(_wi_pool_lock);
+qemu_work_item *curr = atomic_read(_free_pool->head);
+item->next = curr;
+wi_free_pool->head = item;
+qemu_mutex_unlock(_wi_pool_lock);
+}
+
+static qemu_work_item *qemu_wi_pool_remove(void)
+{
+qemu_mutex_lock(_wi_pool_lock);
+qemu_work_item *curr = atomic_read(_free_pool->head);
+if (curr == NULL) {
+goto out;
+}
+wi_free_pool->head = curr->next;
+curr->next = NULL;
+
+ out:
+qemu_mutex_unlock(_wi_pool_lock);
+return curr;
+}
+
 void qemu_init_cpu_list(void)
 {
 /* This is needed because qemu_init_cpu_list is also called by the
@@ -43,6 +87,9 @@ void qemu_init_cpu_list(void)
 qemu_cond_init(_cond);
 qemu_cond_init(_resume);
 qemu_cond_init(_work_cond);
+
+qemu_init_workitem_pool();
+qemu_mutex_init(_wi_pool_lock);
 }
 
 void cpu_list_lock(void)
@@ -106,14 +153,7 @@ void cpu_list_remove(CPUState *cpu)
 qemu_mutex_unlock(_cpu_list_lock);
 }
 
-struct qemu_work_item {
-struct qemu_work_item *next;
-run_on_cpu_func func;
-run_on_cpu_data data;
-bool free, exclusive, done;
-};
-
-static void queue_work_on_cpu(CPUState *cpu, struct qemu_work_item *wi)
+static void queue_work_on_cpu(CPUState *cpu, qemu_work_item *wi)
 {
 qemu_mutex_lock(>work_mutex);
 if (cpu->queued_work_first == NULL) {
@@ -132,7 +172,7 @@ static void queue_work_on_cpu(CPUState *cpu, struct 
qemu_work_item *wi)
 void do_run_on_cpu(CPUState *cpu, run_on_cpu_func func, run_on_cpu_data data,
QemuMutex *mutex)
 {
-struct qemu_work_item wi;
+qemu_work_item wi;
 
 if (qemu_cpu_is_self(cpu)) {
 func(cpu, data);
@@ -156,9 +196,11 @@ void do_run_on_cpu(CPUState *cpu, run_on_cpu_func func, 
run_on_cpu_data data,
 
 void async_run_on_cpu(CPUState *cpu, run_on_cpu_func func, run_on_cpu_data 
data)
 {
-struct qemu_work_item *wi;
+qemu_work_item *wi = qemu_wi_pool_remove();
 
-wi = g_malloc0(sizeof(struct qemu_work_item));
+if (!wi) {
+wi = g_malloc0(sizeof(qemu_work_item));
+}
 wi->func = func;
 wi->data = data;
 wi->free = true;
@@ -299,9 +341,11 @@ void cpu_exec_end(CPUState *cpu)
 void async_safe_run_on_cpu(CPUState *cpu, run_on_cpu_func func,
run_on_cpu_data data)
 {
-struct qemu_work_item *wi;
+qemu_work_item *wi = qemu_wi_pool_remove();
 
-wi = g_malloc0(sizeof(struct qemu_work_item));
+if (!wi) {
+wi = g_malloc0(sizeof(qemu_work_item));
+}
 wi->func = func;
 wi->data = data;
 wi->free = true;
@@ -312,7 +356,7 @@ void async_safe_run_on_cpu(CPUState *cpu, run_on_cpu_func 
func,
 
 void process_queued_cpu_work(CPUState *cpu)
 {
-struct qemu_work_item *wi;
+qemu_work_item *wi;
 
 if (cpu->queued_work_first == NULL) {
 return;
@@ -343,7 +387,8 @@ void process_queued_cpu_work(CPUState *cpu)
 }
 qemu_mutex_lock(>work_mutex);
 if (wi->free) {
-g_free(wi);
+memset(wi, 0, sizeof(qemu_work_item));
+qemu_wi_pool_insert(wi);
 } else {
 atomic_mb_set(>done, true);
 }
-- 
2.13.0