> On 04/15/14 01:55, Michael R. Hines wrote:
>> 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 <ak...@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
>
> I have no idea how I managed to miss that.
>
>> - 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?
>
> The patch doesn't change buffer_is_zero() internally, so callers of
> buffer_is_zero() are unaffected. The patch turns two indirect callers of
> buffer_find_nonzero_offset() into two "differently indirect" callers of
> the same (which I now see thanks to your explanation). Hence,
>
> Before:
> ram_save_block -> is_zero_range -> buffer_find_nonzero_offset
> ram_handle_compressed -> is_zero_range -> buffer_find_nonzero_offset
Because of *static inline *is_zero_range, it may be like that:
ram_save_block -> buffer_find_nonzero_offset
ram_handle_compressed -> buffer_find_nonzero_offset
But this affects little in my testing.
Best regards
ChenLiang
> After:
> ram_save_block -> buffer_is_zero -> buffer_find_nonzero_offset
> ram_handle_compressed -> buffer_is_zero -> buffer_find_nonzero_offset
>
> Reviewed-by: Laszlo Ersek <ler...@redhat.com>
>
> Thanks!
> Laszlo
