Re: [Qemu-devel] [QEMU 1/7] balloon: speed up inflating & deflating process

[Thomas Huth]

On 14.06.2016 16:41, Li, Liang Z wrote:
>>> On 13.06.2016 12:16, Liang Li wrote:
>>>> The implementation of the current virtio-balloon is not very
>>>> efficient, Bellow is test result of time spends on inflating the
>>>> balloon to 3GB of a 4GB idle guest:
>>>>
>>>> a. allocating pages (6.5%, 103ms)
>>>> b. sending PFNs to host (68.3%, 787ms) c. address translation (6.1%,
>>>> 96ms) d. madvise (19%, 300ms)
>>>>
>>>> It takes about 1577ms for the whole inflating process to complete.
>>>> The test shows that the bottle neck is the stage b and stage d.
>>>>
>>>> If using a bitmap to send the page info instead of the PFNs, we can
>>>> reduce the overhead spends on stage b quite a lot. Furthermore, it's
>>>> possible to do the address translation and do the madvise with a
>>>> bulk of pages, instead of the current page per page way, so the
>>>> overhead of stage c and stage d can also be reduced a lot.
>>>>
>>>> This patch is the QEMU side implementation which is intended to
>>>> speed up the inflating & deflating process by adding a new feature
>>>> to the virtio-balloon device. And now, inflating the balloon to 3GB
>>>> of a 4GB idle guest only takes 210ms, it's about 8 times as fast as before.
[...]
>>>> +            chunk = TARGET_PAGE_SIZE;
>>>> +        }
>>>> +    }
>>>> +}
>>>> +
>>>> +static void balloon_bulk_pages(ram_addr_t base_pfn, unsigned long
>>> *bitmap,
>>>> +                               unsigned long len, int page_shift,
>>>> +bool deflate) { #if defined(__linux__)
>>>
>>> Why do you need this #if here?
>>>
>> Ooh,  it is wrong to add the '#if' here, will remove.
>>
> No, it is needed, just follow the code in balloon_page().
> only Linux support the madvise().

I think it is not needed anymore today and the #if in balloon_page could
be removed, too: As far as I can see, the #if there is from the early
days, when there was no wrapper around madvise() yet. But nowadays,
we've got the qemu_madvise() wrapper which takes care of either using
madvise(), posix_madvise() or doing nothing, so the virtio-balloon code
should be able to work without the #if now.

 Thomas