Re: [Qemu-trivial] [PATCH] arch_init.c: remove duplicate function

["Michael R. Hines" <mrhines@linux.vnet.ibm.com>]

On 04/14/2014 05:19 PM, Laszlo Ersek wrote:
> On 04/14/14 04:27, Amos Kong wrote:
>> We already have a function buffer_is_zero() in util/cutils.c
>>
>> Signed-off-by: Amos Kong <akong@redhat.com>
>> ---
>>   arch_init.c | 9 ++-------
>>   1 file changed, 2 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch_init.c b/arch_init.c
>> index 60c975d..342e5dc 100644
>> --- a/arch_init.c
>> +++ b/arch_init.c
>> @@ -152,11 +152,6 @@ int qemu_read_default_config_files(bool userconfig)
>>       return 0;
>>   }
>>   
>> -static inline bool is_zero_range(uint8_t *p, uint64_t size)
>> -{
>> -    return buffer_find_nonzero_offset(p, size) == size;
>> -}
>> -
>>   /* struct contains XBZRLE cache and a static page
>>      used by the compression */
>>   static struct {
>> @@ -587,7 +582,7 @@ static int ram_save_block(QEMUFile *f, bool last_stage)
>>                           acct_info.dup_pages++;
>>                       }
>>                   }
>> -            } else if (is_zero_range(p, TARGET_PAGE_SIZE)) {
>> +            } else if (buffer_is_zero(p, TARGET_PAGE_SIZE)) {
>>                   acct_info.dup_pages++;
>>                   bytes_sent = save_block_hdr(f, block, offset, cont,
>>                                               RAM_SAVE_FLAG_COMPRESS);
>> @@ -983,7 +978,7 @@ static inline void *host_from_stream_offset(QEMUFile *f,
>>    */
>>   void ram_handle_compressed(void *host, uint8_t ch, uint64_t size)
>>   {
>> -    if (ch != 0 || !is_zero_range(host, size)) {
>> +    if (ch != 0 || !buffer_is_zero(host, size)) {
>>           memset(host, ch, size);
>>       }
>>   }
>>
> This seems to be correct, functionally -- apparently buffer_is_zero()
> has laxer size requirements than buffer_find_nonzero_offset(). However,
> I think the latter might be faster.
>
> For ram_save_block() I guess the difference is negligible. But
> ram_handle_compressed() is also called from "migration-rdma.c", where I
> can't even guess if a little bit of slowdown would count.
>
> I'm OK with the patch if Michael (CC'd) is.
>
> Thanks
> Laszlo
>

Thanks for the CC.

Actually, it looks like buffer_is_zero() is calling 
buffer_find_nonzero_offset()
as a "first try" anyway - which is the same thing RDMA is doing. So, all
calls to ram_handle_compressed() should hit the branch target there in
buffer_is_zero() for can_use_buffer_find_nonzero_offset() and automatically
drop into the vectorized-optimization to search for zeros, so there 
shouldn't
be any change in performance. The same should apply for TCP migration
as well - it's working on page-granularity, which is always aligned to 
32 or 64 bits.

Paolo? I see that some of the block-migration code and the qemu-img code 
is also
calling buffer_is_zero() - are you guys depending on the performance of any
buffer_is_zero() calls to use the vector-optimized version like 
migration does?

Just for RDMA: Reviewed-by: Michael R. Hines <mrhines@us.ibm.com>

- Michael