Re: [Qemu-devel] [PATCH for-1.5] qom: optimize casting to leaf class and parent class

[Anthony Liguori]

Aurelien Jarno <address@hidden> writes:

> On Fri, May 10, 2013 at 01:47:55PM -0500, Anthony Liguori wrote:
>> Most QOM types use type_register_static but we still strdup the
>> passed data.  However, the original pointers are useful because
>> GCC is pretty good about collapsing strings so its very likely any
>> use of the pointer will end up being that same address.
>> 
>> IOW, with a little trickery, we can compare types by just comparing
>> strings and in fact that's what we do here.
>> 
>> We do this for the two most common cases, casting to a leaf class
>> or to the parent class.
>> 
>> With these two changes, I see a decrease from around 2 hash table
>> lookups to only a thousand with no run time lookups at all.
>> 
>> Cc: Paolo Bonzini <address@hidden>
>> Cc: Aurelien Jarno <address@hidden>
>> Cc: Andreas Färber <address@hidden>
>> Reported-by: Aurelien Jarno <address@hidden>
>> Signed-off-by: Anthony Liguori <address@hidden>
>> ---
>> Aurelien, could you please try this patch with your PPC test case?
>> ---
>>  qom/object.c | 16 ++++++++++++++--
>>  1 file changed, 14 insertions(+), 2 deletions(-)
>> 
>> diff --git a/qom/object.c b/qom/object.c
>> index 75e6aac..5ecfd28 100644
>> --- a/qom/object.c
>> +++ b/qom/object.c
>> @@ -132,7 +132,13 @@ TypeImpl *type_register(const TypeInfo *info)
>>  
>>  TypeImpl *type_register_static(const TypeInfo *info)
>>  {
>> -    return type_register(info);
>> +    TypeImpl *impl;
>> +
>> +    impl = type_register(info);
>> +    impl->name = info->name;
>> +    impl->parent = info->parent;
>> +
>> +    return impl;
>>  }
>>  
>>  static TypeImpl *type_get_by_name(const char *name)
>> @@ -449,10 +455,16 @@ Object *object_dynamic_cast_assert(Object *obj, const 
>> char *typename)
>>  ObjectClass *object_class_dynamic_cast(ObjectClass *class,
>>                                         const char *typename)
>>  {
>> -    TypeImpl *target_type = type_get_by_name(typename);
>> +    TypeImpl *target_type;
>>      TypeImpl *type = class->type;
>>      ObjectClass *ret = NULL;
>>  
>> +    if (type->name == typename || type->parent == typename) {
>> +        return class;
>> +    }
>> +
>> +    target_type = type_get_by_name(typename);
>> +
>>      if (!target_type) {
>>          /* target class type unknown, so fail the cast */
>>          return NULL;
>
> Unfortunately it doesn't fix the problem. I only see a 0.5% improvement,
> which might be in the noise. I still see g_hash_table_lookup and
> g_str_hash quite high in perf top.

I was afraid of this.  I assume the cast comes somewhere other than
where the type was registered.

This patch should address that.  Could you post an image too?  Then I
don't have to keep bugging you with updated patches.

>From 269442decf0f137cf7d09c32442fcd16dd319901 Mon Sep 17 00:00:00 2001
From: Anthony Liguori <address@hidden>
Date: Fri, 10 May 2013 13:43:11 -0500
Subject: [PATCH] qom: optimize casting to leaf class and parent class (v2)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Most QOM types use type_register_static but we still strdup the
passed data.  However, the original pointers are useful because
GCC is pretty good about collapsing strings so its very likely any
use of the pointer will end up being that same address.

IOW, with a little trickery, we can compare types by just comparing
strings and in fact that's what we do here.

We do this for the two most common cases, casting to a leaf class
or to the parent class.

With these two changes, I see a decrease from around 2 hash table
lookups to only a thousand with no run time lookups at all.

Cc: Paolo Bonzini <address@hidden>
Cc: Aurelien Jarno <address@hidden>
Cc: Andreas Färber <address@hidden>
Reported-by: Aurelien Jarno <address@hidden>
Signed-off-by: Anthony Liguori <address@hidden>
---
v1 -> v2:
 - use strcmp() instead of ==
---
 qom/object.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/qom/object.c b/qom/object.c
index 75e6aac..99b5b33 100644
--- a/qom/object.c
+++ b/qom/object.c
@@ -132,7 +132,13 @@ TypeImpl *type_register(const TypeInfo *info)
 
 TypeImpl *type_register_static(const TypeInfo *info)
 {
-    return type_register(info);
+    TypeImpl *impl;
+
+    impl = type_register(info);
+    impl->name = info->name;
+    impl->parent = info->parent;
+
+    return impl;
 }
 
 static TypeImpl *type_get_by_name(const char *name)
@@ -449,10 +455,17 @@ Object *object_dynamic_cast_assert(Object *obj, const 
char *typename)
 ObjectClass *object_class_dynamic_cast(ObjectClass *class,
                                        const char *typename)
 {
-    TypeImpl *target_type = type_get_by_name(typename);
+    TypeImpl *target_type;
     TypeImpl *type = class->type;
     ObjectClass *ret = NULL;
 
+    if (strcmp(type->name, typename) == 0 ||
+        strcmp(type->parent, typename) == 0) {
+        return class;
+    }
+
+    target_type = type_get_by_name(typename);
+
     if (!target_type) {
         /* target class type unknown, so fail the cast */
         return NULL;
-- 
1.8.0

Regards,

Anthony Liguori

>
> -- 
> Aurelien Jarno                                GPG: 1024D/F1BCDB73
> address@hidden                 http://www.aurel32.net