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| From: | Wenchao Xia |
| Subject: | Re: [Qemu-devel] broken incoming migration |
| Date: | Sun, 09 Jun 2013 11:01:14 +0800 |
| User-agent: | Mozilla/5.0 (Windows NT 5.1; rv:17.0) Gecko/20130509 Thunderbird/17.0.6 |
于 2013-6-9 10:34, Alexey Kardashevskiy 写道:
On 06/09/2013 12:16 PM, Wenchao Xia wrote:于 2013-6-8 16:30, Alexey Kardashevskiy 写道:On 06/08/2013 06:27 PM, Wenchao Xia wrote: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. :) Paolothe 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?How would the destination guest know if some page must be cleared? The previous patch (which Peter reverted) did not send anything for the pages which were zero on the source side.If an page was not received and destination knows that page should exist according to total size, fill it with zero at destination, would it solve the problem?It is _live_ migration, the source sends changes, same pages can change and be sent several times. So we would need to turn tracking on on the destination to know if some page was received from the source or changed by the destination itself (by writing there bios/firmware images, etc) and then clear pages which were touched by the destination and were not sent by the source.
OK, I can understand the problem is, for example: Destination boots up with 0x0000-0xFFFF filled with bios image. Source forgot to send zero pages in 0x0000-0xFFFF. After migration destination got 0x0000-0xFFFF dirty(different with source) Thanks for explain. This seems refer to the migration protocol: how should the guest treat unsent pages. The patch causing the problem, actually treat zero pages as "not to sent" at source, but another half is missing: treat "not received" as zero pages at destination. I guess if second half is added, problem is gone: after page transfer completed, before destination resume, fill zero in "not received" pages.
Or we do not make guesses, the source sends everything and the destination simply checks if a page which is empty on the source is empty on the destination and avoid writing zeroes to it. Looks simpler to me and this is what the new patch does.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
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