migration/multifd-zero-page.c | 4 +++- migration/multifd-zlib.c | 1 + migration/multifd-zstd.c | 1 + migration/multifd.c | 1 + migration/ram.c | 4 ++++ migration/ram.h | 1 + 6 files changed, 11 insertions(+), 1 deletion(-)
1. Description of multiple page faults for received zero pages a. -mem-prealloc feature and hugepage backend are not enabled on the destination b. After receiving the zero pages, the destination first determines if the current page content is 0 via buffer_is_zero, this may cause a read page fault perf record -e page-faults information below 13.75% 13.75% multifdrecv_0 qemu-system-x86_64 [.] buffer_zero_avx512 11.85% 11.85% multifdrecv_1 qemu-system-x86_64 [.] buffer_zero_avx512 multifd_recv_thread nocomp_recv multifd_recv_zero_page_process buffer_is_zero select_accel_fn buffer_zero_avx512 c. Other page faults mainly come from writing operations to normal and zero pages. 2. Solution a. During the multifd migration process, the received pages are tracked through RAMBlock's receivedmap. b. If received zero page is not set in recvbitmap, the destination will not check whether the page content is 0, thus avoiding the occurrence of read fault. c. If the zero page has been set in receivedmap, set the page with 0 directly. There are two reasons for this 1. It's unlikely a zero page if it's sent once or more. 2. For the 1st time destination received a zero page, it must be a zero page, so no need to scan for the 1st round. 3. Test Result 16 vCPUs and 64G memory VM, multifd number is 2, and 100G network bandwidth 3.1 Test case: 16 vCPUs are idle and only 2G memory are used +-----------+--------+--------+----------+ |MultiFD | total |downtime| Page | |Nocomp | time | | Faults | | | (ms) | (ms) | | +-----------+--------+--------+----------+ |with | | | | |recvbitmap | 7335| 180| 2716| +-----------+--------+--------+----------+ |without | | | | |recvbitmap | 7771| 153| 121357| +-----------+--------+--------+----------+ +-----------+--------+--------+--------+-------+--------+-------------+ |MultiFD | total |downtime| SVM |SVM | IOTLB | IO PageFault| |QPL | time | | IO TLB |IO Page| MaxTime| MaxTime | | | (ms) | (ms) | Flush |Faults | (us) | (us) | +-----------+--------+--------+--------+-------+--------+-------------+ |with | | | | | | | |recvbitmap | 10224| 175| 410| 27429| 1| 447| +-----------+--------+--------+--------+-------+--------+-------------+ |without | | | | | | | |recvbitmap | 11253| 153| 80756| 38655| 25| 18349| +-----------+--------+--------+--------+-------+--------+-------------+ 3.2 Test case: 16 vCPUs are idle and 56G memory(not zero) are used +-----------+--------+--------+----------+ |MultiFD | total |downtime| Page | |Nocomp | time | | Faults | | | (ms) | (ms) | | +-----------+--------+--------+----------+ |with | | | | |recvbitmap | 16825| 165| 52967| +-----------+--------+--------+----------+ |without | | | | |recvbitmap | 12987| 159| 2672677| +-----------+--------+--------+----------+ +-----------+--------+--------+--------+-------+--------+-------------+ |MultiFD | total |downtime| SVM |SVM | IOTLB | IO PageFault| |QPL | time | | IO TLB |IO Page| MaxTime| MaxTime | | | (ms) | (ms) | Flush |Faults | (us) | (us) | +-----------+--------+--------+--------+-------+--------+-------------+ |with | | | | | | | |recvbitmap | 132315| 77| 890| 937105| 60| 9581| +-----------+--------+--------+--------+-------+--------+-------------+ |without | | | | | | | |recvbitmap | >138333| N/A| 1647701| 981899| 43| 21018| +-----------+--------+--------+--------+-------+--------+-------------+ From the test result, both of page faults and IOTLB Flush operations can be significantly reduced. The reason is that zero page processing does not trigger read faults, and a large number of zero pages do not even trigger write faults (Test 3.1), because it is considered that after the destination is started, the content of unaccessed pages is 0. I have a concern here, the RAM memory is allocated by mmap with anonymous flag, and if the first received zero page is not set to 0 explicitly, does this ensure that the received zero pages memory data is 0? In this case, the performance impact of live migration is not big because the destination is not the bottleneck. When using QPL (SVM-capable device), even if IOTLB is improved, the overall performance will still be seriously degraded because a large number of IO page faults are still generated. Previous discussion link: 1. https://lore.kernel.org/all/CAAYibXib+TWnJpV22E=adncdBmwXJRqgRjJXK7X71J=bDfaxDg@mail.gmail.com/ 2. https://lore.kernel.org/all/PH7PR11MB594123F7EEFEBFCE219AF100A33A2@PH7PR11MB5941.namprd11.prod.outlook.com/ Yuan Liu (1): migration/multifd: solve zero page causing multiple page faults migration/multifd-zero-page.c | 4 +++- migration/multifd-zlib.c | 1 + migration/multifd-zstd.c | 1 + migration/multifd.c | 1 + migration/ram.c | 4 ++++ migration/ram.h | 1 + 6 files changed, 11 insertions(+), 1 deletion(-) -- 2.39.3
> -----Original Message----- > From: Liu, Yuan1 <yuan1.liu@intel.com> > Sent: Monday, April 1, 2024 11:41 PM > To: peterx@redhat.com; farosas@suse.de > Cc: qemu-devel@nongnu.org; hao.xiang@bytedance.com; > bryan.zhang@bytedance.com; Liu, Yuan1 <yuan1.liu@intel.com>; Zou, Nanhai > <nanhai.zou@intel.com> > Subject: [PATCH 0/1] Solve zero page causing multiple page faults > > 1. Description of multiple page faults for received zero pages > a. -mem-prealloc feature and hugepage backend are not enabled on the > destination > b. After receiving the zero pages, the destination first determines if > the current page content is 0 via buffer_is_zero, this may cause a > read page fault > > perf record -e page-faults information below > 13.75% 13.75% multifdrecv_0 qemu-system-x86_64 [.] > buffer_zero_avx512 > 11.85% 11.85% multifdrecv_1 qemu-system-x86_64 [.] > buffer_zero_avx512 > multifd_recv_thread > nocomp_recv > multifd_recv_zero_page_process > buffer_is_zero > select_accel_fn > buffer_zero_avx512 > > c. Other page faults mainly come from writing operations to normal and > zero pages. > > 2. Solution > a. During the multifd migration process, the received pages are > tracked > through RAMBlock's receivedmap. > > b. If received zero page is not set in recvbitmap, the destination > will not > check whether the page content is 0, thus avoiding the occurrence > of > read fault. > > c. If the zero page has been set in receivedmap, set the page with 0 > directly. > > There are two reasons for this > 1. It's unlikely a zero page if it's sent once or more. > 2. For the 1st time destination received a zero page, it must be a > zero > page, so no need to scan for the 1st round. > > 3. Test Result 16 vCPUs and 64G memory VM, multifd number is 2, > and 100G network bandwidth > > 3.1 Test case: 16 vCPUs are idle and only 2G memory are used > +-----------+--------+--------+----------+ > |MultiFD | total |downtime| Page | > |Nocomp | time | | Faults | > | | (ms) | (ms) | | > +-----------+--------+--------+----------+ > |with | | | | > |recvbitmap | 7335| 180| 2716| > +-----------+--------+--------+----------+ > |without | | | | > |recvbitmap | 7771| 153| 121357| > +-----------+--------+--------+----------+ > > +-----------+--------+--------+--------+-------+--------+------------- > + > |MultiFD | total |downtime| SVM |SVM | IOTLB | IO > PageFault| > |QPL | time | | IO TLB |IO Page| MaxTime| MaxTime > | > | | (ms) | (ms) | Flush |Faults | (us) | (us) > | > +-----------+--------+--------+--------+-------+--------+------------- > + > |with | | | | | | > | > |recvbitmap | 10224| 175| 410| 27429| 1| > 447| > +-----------+--------+--------+--------+-------+--------+------------- > + > |without | | | | | | > | > |recvbitmap | 11253| 153| 80756| 38655| 25| > 18349| > +-----------+--------+--------+--------+-------+--------+------------- > + > > > 3.2 Test case: 16 vCPUs are idle and 56G memory(not zero) are used > +-----------+--------+--------+----------+ > |MultiFD | total |downtime| Page | > |Nocomp | time | | Faults | > | | (ms) | (ms) | | > +-----------+--------+--------+----------+ > |with | | | | > |recvbitmap | 16825| 165| 52967| > +-----------+--------+--------+----------+ > |without | | | | > |recvbitmap | 12987| 159| 2672677| > +-----------+--------+--------+----------+ > > +-----------+--------+--------+--------+-------+--------+------------- > + > |MultiFD | total |downtime| SVM |SVM | IOTLB | IO > PageFault| > |QPL | time | | IO TLB |IO Page| MaxTime| MaxTime > | > | | (ms) | (ms) | Flush |Faults | (us) | (us) > | > +-----------+--------+--------+--------+-------+--------+------------- > + > |with | | | | | | > | > |recvbitmap | 132315| 77| 890| 937105| 60| > 9581| > +-----------+--------+--------+--------+-------+--------+------------- > + > |without | | | | | | > | > |recvbitmap | >138333| N/A| 1647701| 981899| 43| > 21018| > +-----------+--------+--------+--------+-------+--------+------------- > + > > > From the test result, both of page faults and IOTLB Flush operations can > be significantly reduced. The reason is that zero page processing does not > trigger read faults, and a large number of zero pages do not even trigger > write faults (Test 3.1), because it is considered that after the > destination > is started, the content of unaccessed pages is 0. > > I have a concern here, the RAM memory is allocated by mmap with anonymous > flag, and if the first received zero page is not set to 0 explicitly, does > this ensure that the received zero pages memory data is 0? I got the answer here MAP_ANONYMOUS The mapping is not backed by any file; its contents are initialized to zero. The fd argument is ignored; however, some implementations require fd to be -1 if MAP_ANONYMOUS (or MAP_ANON) is specified, and porta�\ ble applications should ensure this. The offset argument should be zero. The use of MAP_ANONYMOUS in conjunction with MAP_SHARED is supported on Linux only since kernel 2.4. > In this case, the performance impact of live migration is not big > because the destination is not the bottleneck. > > When using QPL (SVM-capable device), even if IOTLB is improved, the > overall performance will still be seriously degraded because a large > number of IO page faults are still generated. > > Previous discussion link: > 1. > https://lore.kernel.org/all/CAAYibXib+TWnJpV22E=adncdBmwXJRqgRjJXK7X71J=bD > faxDg@mail.gmail.com/ > 2. > https://lore.kernel.org/all/PH7PR11MB594123F7EEFEBFCE219AF100A33A2@PH7PR11 > MB5941.namprd11.prod.outlook.com/ > > Yuan Liu (1): > migration/multifd: solve zero page causing multiple page faults > > migration/multifd-zero-page.c | 4 +++- > migration/multifd-zlib.c | 1 + > migration/multifd-zstd.c | 1 + > migration/multifd.c | 1 + > migration/ram.c | 4 ++++ > migration/ram.h | 1 + > 6 files changed, 11 insertions(+), 1 deletion(-) > > -- > 2.39.3
© 2016 - 2024 Red Hat, Inc.