[Qemu-devel] [PATCH] migration: cache memory region ram ptr

[Qemu-devel] [PATCH] migration: cache memory region ram ptr

[Peter Lieven]

we currently look up the ram ptr for each single page. Cache
the pointer while we operate on the same block.

Signed-off-by: Peter Lieven <pl@kamp.de>
---
 arch_init.c |   23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/arch_init.c b/arch_init.c
index 582b716..ce338aa 100644
--- a/arch_init.c
+++ b/arch_init.c
@@ -594,13 +594,19 @@ static int ram_save_block(QEMUFile *f, bool last_stage)
                 ram_bulk_stage = false;
             }
         } else {
+            static uint8_t *ram_ptr;
             int ret;
             uint8_t *p;
             bool send_async = true;
-            int cont = (block == last_sent_block) ?
-                RAM_SAVE_FLAG_CONTINUE : 0;
+            int cont = 0;
 
-            p = memory_region_get_ram_ptr(mr) + offset;
+            if (block != last_sent_block) {
+                ram_ptr = memory_region_get_ram_ptr(mr);
+            } else {
+                cont = RAM_SAVE_FLAG_CONTINUE;
+            }
+
+            p = ram_ptr + offset;
 
             /* In doubt sent page as normal */
             bytes_sent = -1;
@@ -990,16 +996,17 @@ static inline void *host_from_stream_offset(QEMUFile *f,
                                             int flags)
 {
     static RAMBlock *block = NULL;
+    static uint8_t *ram_ptr;
     char id[256];
     uint8_t len;
 
     if (flags & RAM_SAVE_FLAG_CONTINUE) {
-        if (!block) {
+        if (!block || !ram_ptr) {
             fprintf(stderr, "Ack, bad migration stream!\n");
             return NULL;
         }
 
-        return memory_region_get_ram_ptr(block->mr) + offset;
+        return ram_ptr + offset;
     }
 
     len = qemu_get_byte(f);
@@ -1007,8 +1014,10 @@ static inline void *host_from_stream_offset(QEMUFile *f,
     id[len] = 0;
 
     QTAILQ_FOREACH(block, &ram_list.blocks, next) {
-        if (!strncmp(id, block->idstr, sizeof(id)))
-            return memory_region_get_ram_ptr(block->mr) + offset;
+        if (!strncmp(id, block->idstr, sizeof(id))) {
+            ram_ptr = memory_region_get_ram_ptr(block->mr);
+            return ram_ptr + offset;
+        }
     }
 
     fprintf(stderr, "Can't find block %s!\n", id);
-- 
1.7.9.5

Re: [Qemu-devel] [PATCH] migration: cache memory region ram ptr

[陈梁]

Hi,
The patch is correct. There is a small improved point.
> 
>             /* In doubt sent page as normal */
>             bytes_sent = -1;
> @@ -990,16 +996,17 @@ static inline void *host_from_stream_offset(QEMUFile *f,
>                                             int flags)
> {
>     static RAMBlock *block = NULL;
       RAMBlock *block = NULL;

> +    static uint8_t *ram_ptr;
>     char id[256];
>     uint8_t len;
> 
>     if (flags & RAM_SAVE_FLAG_CONTINUE) {
> -        if (!block) {
> +        if (!block || !ram_ptr) {

             if (!ram_ptr) {

Best regards
ChenLiang

>             fprintf(stderr, "Ack, bad migration stream!\n");
>             return NULL;
>         }
> 
> -        return memory_region_get_ram_ptr(block->mr) + offset;
> +        return ram_ptr + offset;
>     }
> 
>     len = qemu_get_byte(f);
> @@ -1007,8 +1014,10 @@ static inline void *host_from_stream_offset(QEMUFile *f,
>     id[len] = 0;
> 
>     QTAILQ_FOREACH(block, &ram_list.blocks, next) {
> -        if (!strncmp(id, block->idstr, sizeof(id)))
> -            return memory_region_get_ram_ptr(block->mr) + offset;
> +        if (!strncmp(id, block->idstr, sizeof(id))) {
> +            ram_ptr = memory_region_get_ram_ptr(block->mr);
> +            return ram_ptr + offset;
> +        }
>     }
> 
>     fprintf(stderr, "Can't find block %s!\n", id);
> -- 
> 1.7.9.5
> 
> 

Re: [Qemu-devel] [PATCH] migration: cache memory region ram ptr

[Peter Lieven]

Am 10.05.2014 13:38, schrieb 陈梁:
> Hi,
> The patch is correct. There is a small improved point.
>>             /* In doubt sent page as normal */
>>             bytes_sent = -1;
>> @@ -990,16 +996,17 @@ static inline void *host_from_stream_offset(QEMUFile *f,
>>                                             int flags)
>> {
>>     static RAMBlock *block = NULL;
>        RAMBlock *block = NULL;

This one has to be static as well. Why do you think it should not?

Peter

>
>> +    static uint8_t *ram_ptr;
>>     char id[256];
>>     uint8_t len;
>>
>>     if (flags & RAM_SAVE_FLAG_CONTINUE) {
>> -        if (!block) {
>> +        if (!block || !ram_ptr) {
>              if (!ram_ptr) {
>
> Best regards
> ChenLiang
>
>>             fprintf(stderr, "Ack, bad migration stream!\n");
>>             return NULL;
>>         }
>>
>> -        return memory_region_get_ram_ptr(block->mr) + offset;
>> +        return ram_ptr + offset;
>>     }
>>
>>     len = qemu_get_byte(f);
>> @@ -1007,8 +1014,10 @@ static inline void *host_from_stream_offset(QEMUFile *f,
>>     id[len] = 0;
>>
>>     QTAILQ_FOREACH(block, &ram_list.blocks, next) {
>> -        if (!strncmp(id, block->idstr, sizeof(id)))
>> -            return memory_region_get_ram_ptr(block->mr) + offset;
>> +        if (!strncmp(id, block->idstr, sizeof(id))) {
>> +            ram_ptr = memory_region_get_ram_ptr(block->mr);
>> +            return ram_ptr + offset;
>> +        }
>>     }
>>
>>     fprintf(stderr, "Can't find block %s!\n", id);
>> -- 
>> 1.7.9.5
>>
>>

Re: [Qemu-devel] [PATCH] migration: cache memory region ram ptr

[Paolo Bonzini]

Il 10/05/2014 12:51, Peter Lieven ha scritto:
> we currently look up the ram ptr for each single page. Cache
> the pointer while we operate on the same block.

Why don't you instead cache the result in the MemoryRegion, so that 
memory_region_get_ram_ptr becomes a simple, inline field access?

Paolo

> Signed-off-by: Peter Lieven <pl@kamp.de>
> ---
>  arch_init.c |   23 ++++++++++++++++-------
>  1 file changed, 16 insertions(+), 7 deletions(-)
>
> diff --git a/arch_init.c b/arch_init.c
> index 582b716..ce338aa 100644
> --- a/arch_init.c
> +++ b/arch_init.c
> @@ -594,13 +594,19 @@ static int ram_save_block(QEMUFile *f, bool last_stage)
>                  ram_bulk_stage = false;
>              }
>          } else {
> +            static uint8_t *ram_ptr;
>              int ret;
>              uint8_t *p;
>              bool send_async = true;
> -            int cont = (block == last_sent_block) ?
> -                RAM_SAVE_FLAG_CONTINUE : 0;
> +            int cont = 0;
>
> -            p = memory_region_get_ram_ptr(mr) + offset;
> +            if (block != last_sent_block) {
> +                ram_ptr = memory_region_get_ram_ptr(mr);
> +            } else {
> +                cont = RAM_SAVE_FLAG_CONTINUE;
> +            }
> +
> +            p = ram_ptr + offset;
>
>              /* In doubt sent page as normal */
>              bytes_sent = -1;
> @@ -990,16 +996,17 @@ static inline void *host_from_stream_offset(QEMUFile *f,
>                                              int flags)
>  {
>      static RAMBlock *block = NULL;
> +    static uint8_t *ram_ptr;
>      char id[256];
>      uint8_t len;
>
>      if (flags & RAM_SAVE_FLAG_CONTINUE) {
> -        if (!block) {
> +        if (!block || !ram_ptr) {
>              fprintf(stderr, "Ack, bad migration stream!\n");
>              return NULL;
>          }
>
> -        return memory_region_get_ram_ptr(block->mr) + offset;
> +        return ram_ptr + offset;
>      }
>
>      len = qemu_get_byte(f);
> @@ -1007,8 +1014,10 @@ static inline void *host_from_stream_offset(QEMUFile *f,
>      id[len] = 0;
>
>      QTAILQ_FOREACH(block, &ram_list.blocks, next) {
> -        if (!strncmp(id, block->idstr, sizeof(id)))
> -            return memory_region_get_ram_ptr(block->mr) + offset;
> +        if (!strncmp(id, block->idstr, sizeof(id))) {
> +            ram_ptr = memory_region_get_ram_ptr(block->mr);
> +            return ram_ptr + offset;
> +        }
>      }
>
>      fprintf(stderr, "Can't find block %s!\n", id);
>

Re: [Qemu-devel] [PATCH] migration: cache memory region ram ptr

[Peter Lieven]

You mean,

a) extent the MemoryRegion stuct with a pointer?

b) on the first call to memory_region_get_ram_ptr cache the result in the struct?

Peter

Am 10.05.2014 17:33, schrieb Paolo Bonzini:
> Il 10/05/2014 12:51, Peter Lieven ha scritto:
>> we currently look up the ram ptr for each single page. Cache
>> the pointer while we operate on the same block.
>
> Why don't you instead cache the result in the MemoryRegion, so that memory_region_get_ram_ptr becomes a simple, inline field access?
>
> Paolo
>
>> Signed-off-by: Peter Lieven <pl@kamp.de>
>> ---
>>  arch_init.c |   23 ++++++++++++++++-------
>>  1 file changed, 16 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch_init.c b/arch_init.c
>> index 582b716..ce338aa 100644
>> --- a/arch_init.c
>> +++ b/arch_init.c
>> @@ -594,13 +594,19 @@ static int ram_save_block(QEMUFile *f, bool last_stage)
>>                  ram_bulk_stage = false;
>>              }
>>          } else {
>> +            static uint8_t *ram_ptr;
>>              int ret;
>>              uint8_t *p;
>>              bool send_async = true;
>> -            int cont = (block == last_sent_block) ?
>> -                RAM_SAVE_FLAG_CONTINUE : 0;
>> +            int cont = 0;
>>
>> -            p = memory_region_get_ram_ptr(mr) + offset;
>> +            if (block != last_sent_block) {
>> +                ram_ptr = memory_region_get_ram_ptr(mr);
>> +            } else {
>> +                cont = RAM_SAVE_FLAG_CONTINUE;
>> +            }
>> +
>> +            p = ram_ptr + offset;
>>
>>              /* In doubt sent page as normal */
>>              bytes_sent = -1;
>> @@ -990,16 +996,17 @@ static inline void *host_from_stream_offset(QEMUFile *f,
>>                                              int flags)
>>  {
>>      static RAMBlock *block = NULL;
>> +    static uint8_t *ram_ptr;
>>      char id[256];
>>      uint8_t len;
>>
>>      if (flags & RAM_SAVE_FLAG_CONTINUE) {
>> -        if (!block) {
>> +        if (!block || !ram_ptr) {
>>              fprintf(stderr, "Ack, bad migration stream!\n");
>>              return NULL;
>>          }
>>
>> -        return memory_region_get_ram_ptr(block->mr) + offset;
>> +        return ram_ptr + offset;
>>      }
>>
>>      len = qemu_get_byte(f);
>> @@ -1007,8 +1014,10 @@ static inline void *host_from_stream_offset(QEMUFile *f,
>>      id[len] = 0;
>>
>>      QTAILQ_FOREACH(block, &ram_list.blocks, next) {
>> -        if (!strncmp(id, block->idstr, sizeof(id)))
>> -            return memory_region_get_ram_ptr(block->mr) + offset;
>> +        if (!strncmp(id, block->idstr, sizeof(id))) {
>> +            ram_ptr = memory_region_get_ram_ptr(block->mr);
>> +            return ram_ptr + offset;
>> +        }
>>      }
>>
>>      fprintf(stderr, "Can't find block %s!\n", id);
>>
>

Re: [Qemu-devel] [PATCH] migration: cache memory region ram ptr

[Peter Lieven]

Am 10.05.2014 17:33, schrieb Paolo Bonzini:
> Il 10/05/2014 12:51, Peter Lieven ha scritto:
>> we currently look up the ram ptr for each single page. Cache
>> the pointer while we operate on the same block.
>
> Why don't you instead cache the result in the MemoryRegion, so that memory_region_get_ram_ptr becomes a simple, inline field access?

This seems to work. Wondering if it has other side implications. Basic tests like booting vServers and migration work. What about XEN?

diff --git a/include/exec/memory.h b/include/exec/memory.h
index 1d55ad9..3003875 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -161,6 +161,7 @@ struct MemoryRegion {
     unsigned ioeventfd_nb;
     MemoryRegionIoeventfd *ioeventfds;
     NotifierList iommu_notify;
+    void *ram_ptr;
 };
 
 /**
diff --git a/memory.c b/memory.c
index 3f1df23..78d4032 100644
--- a/memory.c
+++ b/memory.c
@@ -862,6 +862,7 @@ void memory_region_init(MemoryRegion *mr,
     mr->ioeventfd_nb = 0;
     mr->ioeventfds = NULL;
     mr->flush_coalesced_mmio = false;
+    mr->ram_ptr = NULL;
 }
 
 static uint64_t unassigned_mem_read(void *opaque, hwaddr addr,
@@ -1249,7 +1250,12 @@ void *memory_region_get_ram_ptr(MemoryRegion *mr)
 
     assert(mr->terminates);
 
-    return qemu_get_ram_ptr(mr->ram_addr & TARGET_PAGE_MASK);
+    if (mr->ram_ptr) {
+        return mr->ram_ptr;
+    }
+
+    mr->ram_ptr = qemu_get_ram_ptr(mr->ram_addr & TARGET_PAGE_MASK);
+    return mr->ram_ptr;
 }
 
 static void memory_region_update_coalesced_range_as(MemoryRegion *mr, AddressSpace *as)


Peter

>
> Paolo
>
>> Signed-off-by: Peter Lieven <pl@kamp.de>
>> ---
>>  arch_init.c |   23 ++++++++++++++++-------
>>  1 file changed, 16 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch_init.c b/arch_init.c
>> index 582b716..ce338aa 100644
>> --- a/arch_init.c
>> +++ b/arch_init.c
>> @@ -594,13 +594,19 @@ static int ram_save_block(QEMUFile *f, bool last_stage)
>>                  ram_bulk_stage = false;
>>              }
>>          } else {
>> +            static uint8_t *ram_ptr;
>>              int ret;
>>              uint8_t *p;
>>              bool send_async = true;
>> -            int cont = (block == last_sent_block) ?
>> -                RAM_SAVE_FLAG_CONTINUE : 0;
>> +            int cont = 0;
>>
>> -            p = memory_region_get_ram_ptr(mr) + offset;
>> +            if (block != last_sent_block) {
>> +                ram_ptr = memory_region_get_ram_ptr(mr);
>> +            } else {
>> +                cont = RAM_SAVE_FLAG_CONTINUE;
>> +            }
>> +
>> +            p = ram_ptr + offset;
>>
>>              /* In doubt sent page as normal */
>>              bytes_sent = -1;
>> @@ -990,16 +996,17 @@ static inline void *host_from_stream_offset(QEMUFile *f,
>>                                              int flags)
>>  {
>>      static RAMBlock *block = NULL;
>> +    static uint8_t *ram_ptr;
>>      char id[256];
>>      uint8_t len;
>>
>>      if (flags & RAM_SAVE_FLAG_CONTINUE) {
>> -        if (!block) {
>> +        if (!block || !ram_ptr) {
>>              fprintf(stderr, "Ack, bad migration stream!\n");
>>              return NULL;
>>          }
>>
>> -        return memory_region_get_ram_ptr(block->mr) + offset;
>> +        return ram_ptr + offset;
>>      }
>>
>>      len = qemu_get_byte(f);
>> @@ -1007,8 +1014,10 @@ static inline void *host_from_stream_offset(QEMUFile *f,
>>      id[len] = 0;
>>
>>      QTAILQ_FOREACH(block, &ram_list.blocks, next) {
>> -        if (!strncmp(id, block->idstr, sizeof(id)))
>> -            return memory_region_get_ram_ptr(block->mr) + offset;
>> +        if (!strncmp(id, block->idstr, sizeof(id))) {
>> +            ram_ptr = memory_region_get_ram_ptr(block->mr);
>> +            return ram_ptr + offset;
>> +        }
>>      }
>>
>>      fprintf(stderr, "Can't find block %s!\n", id);
>>
>

Re: [Qemu-devel] [PATCH] migration: cache memory region ram ptr

[Peter Lieven]

or even this:

diff --git a/include/exec/memory.h b/include/exec/memory.h
index 1d55ad9..3003875 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -161,6 +161,7 @@ struct MemoryRegion {
     unsigned ioeventfd_nb;
     MemoryRegionIoeventfd *ioeventfds;
     NotifierList iommu_notify;
+    void *ram_ptr;
 };
 
 /**
diff --git a/memory.c b/memory.c
index 3f1df23..6c1e413 100644
--- a/memory.c
+++ b/memory.c
@@ -862,6 +862,7 @@ void memory_region_init(MemoryRegion *mr,
     mr->ioeventfd_nb = 0;
     mr->ioeventfds = NULL;
     mr->flush_coalesced_mmio = false;
+    mr->ram_ptr = NULL;
 }
 
 static uint64_t unassigned_mem_read(void *opaque, hwaddr addr,
@@ -1241,17 +1242,23 @@ void memory_region_reset_dirty(MemoryRegion *mr, hwaddr addr,
     cpu_physical_memory_reset_dirty(mr->ram_addr + addr, size, client);
 }
 
-void *memory_region_get_ram_ptr(MemoryRegion *mr)
+static void *memory_region_get_ram_ptr2(MemoryRegion *mr)
 {
     if (mr->alias) {
         return memory_region_get_ram_ptr(mr->alias) + mr->alias_offset;
     }
-
     assert(mr->terminates);
-
     return qemu_get_ram_ptr(mr->ram_addr & TARGET_PAGE_MASK);
 }
 
+inline void *memory_region_get_ram_ptr(MemoryRegion *mr)
+{
+    if (!mr->ram_ptr) {
+        mr->ram_ptr = memory_region_get_ram_ptr2(mr);
+    }
+    return mr->ram_ptr;
+}
+
 static void memory_region_update_coalesced_range_as(MemoryRegion *mr, AddressSpace *as)
 {
     FlatView *view;


Am 10.05.2014 18:32, schrieb Peter Lieven:
> Am 10.05.2014 17:33, schrieb Paolo Bonzini:
>> Il 10/05/2014 12:51, Peter Lieven ha scritto:
>>> we currently look up the ram ptr for each single page. Cache
>>> the pointer while we operate on the same block.
>> Why don't you instead cache the result in the MemoryRegion, so that memory_region_get_ram_ptr becomes a simple, inline field access?
> This seems to work. Wondering if it has other side implications. Basic tests like booting vServers and migration work. What about XEN?
>
> diff --git a/include/exec/memory.h b/include/exec/memory.h
> index 1d55ad9..3003875 100644
> --- a/include/exec/memory.h
> +++ b/include/exec/memory.h
> @@ -161,6 +161,7 @@ struct MemoryRegion {
>      unsigned ioeventfd_nb;
>      MemoryRegionIoeventfd *ioeventfds;
>      NotifierList iommu_notify;
> +    void *ram_ptr;
>  };
>  
>  /**
> diff --git a/memory.c b/memory.c
> index 3f1df23..78d4032 100644
> --- a/memory.c
> +++ b/memory.c
> @@ -862,6 +862,7 @@ void memory_region_init(MemoryRegion *mr,
>      mr->ioeventfd_nb = 0;
>      mr->ioeventfds = NULL;
>      mr->flush_coalesced_mmio = false;
> +    mr->ram_ptr = NULL;
>  }
>  
>  static uint64_t unassigned_mem_read(void *opaque, hwaddr addr,
> @@ -1249,7 +1250,12 @@ void *memory_region_get_ram_ptr(MemoryRegion *mr)
>  
>      assert(mr->terminates);
>  
> -    return qemu_get_ram_ptr(mr->ram_addr & TARGET_PAGE_MASK);
> +    if (mr->ram_ptr) {
> +        return mr->ram_ptr;
> +    }
> +
> +    mr->ram_ptr = qemu_get_ram_ptr(mr->ram_addr & TARGET_PAGE_MASK);
> +    return mr->ram_ptr;
>  }
>  
>  static void memory_region_update_coalesced_range_as(MemoryRegion *mr, AddressSpace *as)
>
>
> Peter
>
>> Paolo
>>
>>> Signed-off-by: Peter Lieven <pl@kamp.de>
>>> ---
>>>  arch_init.c |   23 ++++++++++++++++-------
>>>  1 file changed, 16 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/arch_init.c b/arch_init.c
>>> index 582b716..ce338aa 100644
>>> --- a/arch_init.c
>>> +++ b/arch_init.c
>>> @@ -594,13 +594,19 @@ static int ram_save_block(QEMUFile *f, bool last_stage)
>>>                  ram_bulk_stage = false;
>>>              }
>>>          } else {
>>> +            static uint8_t *ram_ptr;
>>>              int ret;
>>>              uint8_t *p;
>>>              bool send_async = true;
>>> -            int cont = (block == last_sent_block) ?
>>> -                RAM_SAVE_FLAG_CONTINUE : 0;
>>> +            int cont = 0;
>>>
>>> -            p = memory_region_get_ram_ptr(mr) + offset;
>>> +            if (block != last_sent_block) {
>>> +                ram_ptr = memory_region_get_ram_ptr(mr);
>>> +            } else {
>>> +                cont = RAM_SAVE_FLAG_CONTINUE;
>>> +            }
>>> +
>>> +            p = ram_ptr + offset;
>>>
>>>              /* In doubt sent page as normal */
>>>              bytes_sent = -1;
>>> @@ -990,16 +996,17 @@ static inline void *host_from_stream_offset(QEMUFile *f,
>>>                                              int flags)
>>>  {
>>>      static RAMBlock *block = NULL;
>>> +    static uint8_t *ram_ptr;
>>>      char id[256];
>>>      uint8_t len;
>>>
>>>      if (flags & RAM_SAVE_FLAG_CONTINUE) {
>>> -        if (!block) {
>>> +        if (!block || !ram_ptr) {
>>>              fprintf(stderr, "Ack, bad migration stream!\n");
>>>              return NULL;
>>>          }
>>>
>>> -        return memory_region_get_ram_ptr(block->mr) + offset;
>>> +        return ram_ptr + offset;
>>>      }
>>>
>>>      len = qemu_get_byte(f);
>>> @@ -1007,8 +1014,10 @@ static inline void *host_from_stream_offset(QEMUFile *f,
>>>      id[len] = 0;
>>>
>>>      QTAILQ_FOREACH(block, &ram_list.blocks, next) {
>>> -        if (!strncmp(id, block->idstr, sizeof(id)))
>>> -            return memory_region_get_ram_ptr(block->mr) + offset;
>>> +        if (!strncmp(id, block->idstr, sizeof(id))) {
>>> +            ram_ptr = memory_region_get_ram_ptr(block->mr);
>>> +            return ram_ptr + offset;
>>> +        }
>>>      }
>>>
>>>      fprintf(stderr, "Can't find block %s!\n", id);
>>>

Re: [Qemu-devel] [PATCH] migration: cache memory region ram ptr

[Paolo Bonzini]

Il 10/05/2014 18:32, Peter Lieven ha scritto:
> What about XEN?
>

You're right, Xen wouldn't work.  Your original patch would not break it 
just because Xen doesn't use migration (but the code would be broken).

You would have to cache qemu_get_ram_block rather than qemu_get_ram_ptr, 
move RAMBlock to memory-internal.h, and split the RAMBlock + ram_addr_t 
=> void * conversion out of qemu_get_ram_ptr and into a separate 
function (to be used by memory_region_get_ram_ptr).

I'm not sure of the benefit of your patch though.  qemu_get_ram_block 
already has a 1-item cache, are you seeing a low hit rate there?  Or any 
other profiling that shows qemu_get_ram_ptr as hot?

Paolo

Re: [Qemu-devel] [PATCH] migration: cache memory region ram ptr

[Peter Lieven]

Am 12.05.2014 08:09, schrieb Paolo Bonzini:
> Il 10/05/2014 18:32, Peter Lieven ha scritto:
>> What about XEN?
>>
>
> You're right, Xen wouldn't work.  Your original patch would not break it just because Xen doesn't use migration (but the code would be broken).
>
> You would have to cache qemu_get_ram_block rather than qemu_get_ram_ptr, move RAMBlock to memory-internal.h, and split the RAMBlock + ram_addr_t => void * conversion out of qemu_get_ram_ptr and into a separate function (to be used by memory_region_get_ram_ptr).
>
> I'm not sure of the benefit of your patch though.  qemu_get_ram_block already has a 1-item cache, are you seeing a low hit rate there?  Or any other profiling that shows qemu_get_ram_ptr as hot?

qemu_get_ram_ptr is hot only during migration. But the hit-rate of the LRU cache seems to be good. I am wondering if this is different if the migration
has difficulties to converge, but you might be right it should be neglectible.

I ran some basic migration tests with and without the patch. It might be that the results with the cache are slightly better, but the variance of the
results is high. I had to run a significant number of tests to get more evidence.

Peter

>
> Paolo