Re: [Qemu-devel] broken incoming migration

[Wenchao Xia <xiawenc@linux.vnet.ibm.com>]

 > On 04.06.2013 16:40, Paolo Bonzini wrote:
>> 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
> the following does also not allocate memory, but qemu does...
>
Hi, Peter
   As the patch writes

"not sending zero pages breaks migration if a page is zero
at the source but not at the destination."

   I don't understand why it would be trouble, shouldn't all page
not received in dest be treated as zero pages?

Also, you mean following code is from qemu and it does not allocate
memory with you gcc right? Maybe it is related to KVM, how about
turn off KVM and retry following code in qemu?

> #include <stdio.h>
> #include <stdlib.h>
> #include <assert.h>
> #include <unistd.h>
> #include <sys/resource.h>
> #include <inttypes.h>
> #include <string.h>
> #include <sys/mman.h>
> #include <errno.h>
>
> #if defined __SSE2__
> #include <emmintrin.h>
> #define VECTYPE        __m128i
> #define SPLAT(p)       _mm_set1_epi8(*(p))
> #define ALL_EQ(v1, v2) (_mm_movemask_epi8(_mm_cmpeq_epi8(v1, v2)) ==
> 0xFFFF)
> #else
> #define VECTYPE        unsigned long
> #define SPLAT(p)       (*(p) * (~0UL / 255))
> #define ALL_EQ(v1, v2) ((v1) == (v2))
> #endif
>
> #define BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR 8
>
> /* Round number down to multiple */
> #define QEMU_ALIGN_DOWN(n, m) ((n) / (m) * (m))
>
> /* Round number up to multiple */
> #define QEMU_ALIGN_UP(n, m) QEMU_ALIGN_DOWN((n) + (m) - 1, (m))
>
> #define QEMU_VMALLOC_ALIGN (256 * 4096)
>
> /* alloc shared memory pages */
> void *qemu_anon_ram_alloc(size_t size)
> {
>      size_t align = QEMU_VMALLOC_ALIGN;
>      size_t total = size + align - getpagesize();
>      void *ptr = mmap(0, total, PROT_READ | PROT_WRITE,
>                       MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
>      size_t offset = QEMU_ALIGN_UP((uintptr_t)ptr, align) - (uintptr_t)ptr;
>
>      if (ptr == MAP_FAILED) {
>          fprintf(stderr, "Failed to allocate %zu B: %s\n",
>                  size, strerror(errno));
>          abort();
>      }
>
>      ptr += offset;
>      total -= offset;
>
>      if (offset > 0) {
>          munmap(ptr - offset, offset);
>      }
>      if (total > size) {
>          munmap(ptr + size, total - size);
>      }
>
>      return ptr;
> }
>
> static inline int
> can_use_buffer_find_nonzero_offset(const void *buf, size_t len)
> {
>      return (len % (BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR
>                     * sizeof(VECTYPE)) == 0
>              && ((uintptr_t) buf) % sizeof(VECTYPE) == 0);
> }
>
> size_t buffer_find_nonzero_offset(const void *buf, size_t len)
> {
>      const VECTYPE *p = buf;
>      const VECTYPE zero = (VECTYPE){0};
>      size_t i;
>
>      if (!len) {
>          return 0;
>      }
>
>      assert(can_use_buffer_find_nonzero_offset(buf, len));
>
>      for (i = 0; i < BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR; i++) {
>          if (!ALL_EQ(p[i], zero)) {
>              return i * sizeof(VECTYPE);
>          }
>      }
>
>      for (i = BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR;
>           i < len / sizeof(VECTYPE);
>           i += BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR) {
>          VECTYPE tmp0 = p[i + 0] | p[i + 1];
>          VECTYPE tmp1 = p[i + 2] | p[i + 3];
>          VECTYPE tmp2 = p[i + 4] | p[i + 5];
>          VECTYPE tmp3 = p[i + 6] | p[i + 7];
>          VECTYPE tmp01 = tmp0 | tmp1;
>          VECTYPE tmp23 = tmp2 | tmp3;
>          if (!ALL_EQ(tmp01 | tmp23, zero)) {
>              break;
>          }
>      }
>
>      return i * sizeof(VECTYPE);
> }
>
> int main()
> {
>       //char *x = malloc(1024 << 20);
>       char *x = qemu_anon_ram_alloc(1024 << 20);
>
>       int i, j;
>       int ret = 0;
>       struct rusage rusage;
>       for (i = 0; i < 500; i ++) {
>           for (j = 0; j < 10 << 20; j += 4096) {
>                ret += buffer_find_nonzero_offset((char*) (x + (i << 20)
> + j), 4096);
>           }
>           getrusage( RUSAGE_SELF, &rusage );
>           printf("read offset: %d kB, RSS size: %ld kB", ((i+1) << 10),
> rusage.ru_maxrss);
>           getchar();
>       }
>       printf("%d zero pages\n", ret);
> }
>


-- 
Best Regards

Wenchao Xia