Re: [Qemu-devel] broken incoming migration

[Paolo Bonzini]

Il 04/06/2013 16:38, Peter Lieven ha scritto:
> On 04.06.2013 16:14, Paolo Bonzini wrote:
>> Il 04/06/2013 15:52, Peter Lieven ha scritto:
>>> On 30.05.2013 16:41, Paolo Bonzini wrote:
>>>> Il 30/05/2013 16:38, Peter Lieven ha scritto:
>>>>>>> You could also scan the page for nonzero values before writing it.
>>>>> i had this in mind, but then choosed the other approach.... turned
>>>>> out to be a bad idea.
>>>>>
>>>>> alexey: i will prepare a patch later today, could you then please
>>>>> verify it fixes your problem.
>>>>>
>>>>> paolo: would we still need the madvise or is it enough to not write
>>>>> the zeroes?
>>>> It should be enough to not write them.
>>> Problem: checking the pages for zero allocates them. even at the source.
>> It doesn't look like.  I tried this program and top doesn't show an
>> increasing amount of reserved memory:
>>
>> #include <stdio.h>
>> #include <stdlib.h>
>> int main()
>> {
>>      char *x = malloc(500 << 20);
>>      int i, j;
>>      for (i = 0; i < 500; i += 10) {
>>          for (j = 0; j < 10 << 20; j += 4096) {
>>               *(volatile char*) (x + (i << 20) + j);
>>          }
>>          getchar();
>>      }
>> }
> strange. we are talking about RSS size, right?

None of the three top values change, and only VIRT is >500 MB.

> is the malloc above using mmapped memory?

Yes.

> which kernel version do you use?

3.9.

> what avoids allocating the memory for me is the following (with
> whatever side effects it has ;-))

This would also fail to migrate any page that is swapped out, breaking
overcommit in a more subtle way. :)

Paolo