"Chuang Xu" <[email protected]> writes:
> From: xuchuangxclwt <[email protected]>
>
> In our long-term experience in Bytedance, we've found that under
> the same load, live migration of larger VMs with more devices is
> often more difficult to converge (requiring a larger downtime limit).
>
> Through some testing and calculations, we conclude that bitmap sync time
> affects the calculation of live migration bandwidth.
>
> When the addresses processed are not aligned, a large number of
> clear_dirty ioctl occur (e.g. a 4MB misaligned memory can generate
> 2048 clear_dirty ioctls from two different memory_listener),
> which increases the time required for bitmap_sync and makes it
> more difficult for dirty pages to converge.
>
> For a 64C256G vm with 8 vhost-user-net(32 queue per nic) and
> 16 vhost-user-blk(4 queue per blk), the sync time is as high as *73ms*
> (tested with 10GBps dirty rate, the sync time increases as the dirty
> page rate increases), Here are each part of the sync time:
>
> - sync from kvm to ram_list: 2.5ms
> - vhost_log_sync:3ms
> - sync aligned memory from ram_list to RAMBlock: 5ms
> - sync misaligned memory from ram_list to RAMBlock: 61ms
>
> Attempt to merge those fragmented clear_dirty ioctls, then syncing
> misaligned memory from ram_list to RAMBlock takes only about 1ms,
> and the total sync time is only *12ms*.
>
> Signed-off-by: Chuang Xu <[email protected]>
> ---
> accel/tcg/cputlb.c | 5 ++--
> include/system/physmem.h | 7 +++---
> migration/ram.c | 17 ++++----------
> system/memory.c | 2 +-
> system/physmem.c | 49 ++++++++++++++++++++++++++++------------
> 5 files changed, 47 insertions(+), 33 deletions(-)
>
> diff --git a/accel/tcg/cputlb.c b/accel/tcg/cputlb.c
> index fd1606c856..c8827c8b0d 100644
> --- a/accel/tcg/cputlb.c
> +++ b/accel/tcg/cputlb.c
> @@ -857,8 +857,9 @@ void
> tlb_flush_page_bits_by_mmuidx_all_cpus_synced(CPUState *src_cpu,
> void tlb_protect_code(ram_addr_t ram_addr)
> {
> physical_memory_test_and_clear_dirty(ram_addr & TARGET_PAGE_MASK,
> - TARGET_PAGE_SIZE,
> - DIRTY_MEMORY_CODE);
> + TARGET_PAGE_SIZE,
> + DIRTY_MEMORY_CODE,
> + NULL);
> }
>
> /* update the TLB so that writes in physical page 'phys_addr' are no longer
> diff --git a/include/system/physmem.h b/include/system/physmem.h
> index 879f6eae38..8eeace9d1f 100644
> --- a/include/system/physmem.h
> +++ b/include/system/physmem.h
> @@ -39,9 +39,10 @@ uint64_t physical_memory_set_dirty_lebitmap(unsigned long
> *bitmap,
>
> void physical_memory_dirty_bits_cleared(ram_addr_t start, ram_addr_t length);
>
> -bool physical_memory_test_and_clear_dirty(ram_addr_t start,
> - ram_addr_t length,
> - unsigned client);
> +uint64_t physical_memory_test_and_clear_dirty(ram_addr_t start,
> + ram_addr_t length,
> + unsigned client,
> + unsigned long *dest);
>
> DirtyBitmapSnapshot *
> physical_memory_snapshot_and_clear_dirty(MemoryRegion *mr, hwaddr offset,
> diff --git a/migration/ram.c b/migration/ram.c
> index 29f016cb25..a03c9874a2 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -942,7 +942,6 @@ static uint64_t
> physical_memory_sync_dirty_bitmap(RAMBlock *rb,
> ram_addr_t start,
> ram_addr_t length)
> {
> - ram_addr_t addr;
> unsigned long word = BIT_WORD((start + rb->offset) >> TARGET_PAGE_BITS);
> uint64_t num_dirty = 0;
> unsigned long *dest = rb->bmap;
> @@ -996,17 +995,11 @@ static uint64_t
> physical_memory_sync_dirty_bitmap(RAMBlock *rb,
> } else {
> ram_addr_t offset = rb->offset;
>
> - for (addr = 0; addr < length; addr += TARGET_PAGE_SIZE) {
> - if (physical_memory_test_and_clear_dirty(
> - start + addr + offset,
> - TARGET_PAGE_SIZE,
> - DIRTY_MEMORY_MIGRATION)) {
> - long k = (start + addr) >> TARGET_PAGE_BITS;
> - if (!test_and_set_bit(k, dest)) {
> - num_dirty++;
> - }
> - }
> - }
> + num_dirty = physical_memory_test_and_clear_dirty(
> + start + offset,
> + length,
> + DIRTY_MEMORY_MIGRATION,
> + dest);
> }
>
> return num_dirty;
> diff --git a/system/memory.c b/system/memory.c
> index 8b84661ae3..666364392d 100644
> --- a/system/memory.c
> +++ b/system/memory.c
> @@ -2424,7 +2424,7 @@ void memory_region_reset_dirty(MemoryRegion *mr, hwaddr
> addr,
> {
> assert(mr->ram_block);
> physical_memory_test_and_clear_dirty(
> - memory_region_get_ram_addr(mr) + addr, size, client);
> + memory_region_get_ram_addr(mr) + addr, size, client, NULL);
> }
>
> int memory_region_get_fd(MemoryRegion *mr)
> diff --git a/system/physmem.c b/system/physmem.c
> index c9869e4049..f8b660dafe 100644
> --- a/system/physmem.c
> +++ b/system/physmem.c
> @@ -1089,19 +1089,31 @@ void physical_memory_set_dirty_range(ram_addr_t
> start, ram_addr_t length,
> }
> }
>
> -/* Note: start and end must be within the same ram block. */
> -bool physical_memory_test_and_clear_dirty(ram_addr_t start,
> +/*
> + * Note: start and end must be within the same ram block.
> + *
> + * @dest usage:
I'm not sure if it's just me, but I find this "dest" term quite
confusing. "bmap" might be more straight-forward.
> + * - When @dest is provided, set bits for newly discovered dirty pages
> + * only if the bit wasn't already set in dest, and count those pages
> + * in num_dirty.
Am I misreading the code? I don't see this "set ... only if the bit
wasn't already set" part. What I see is: "set bits, but only count those
pages if the bit wasn't already set".
> + * - When @dest is NULL, count all dirty pages in the range
> + *
> + * @return:
> + * - Number of dirty guest pages found within [start, start + length).
> + */
> +uint64_t physical_memory_test_and_clear_dirty(ram_addr_t start,
> ram_addr_t length,
> - unsigned client)
> + unsigned client,
> + unsigned long *dest)
> {
> DirtyMemoryBlocks *blocks;
> unsigned long end, page, start_page;
> - bool dirty = false;
> + uint64_t num_dirty = 0;
> RAMBlock *ramblock;
> uint64_t mr_offset, mr_size;
>
> if (length == 0) {
> - return false;
> + return 0;
> }
>
> end = TARGET_PAGE_ALIGN(start + length) >> TARGET_PAGE_BITS;
> @@ -1118,12 +1130,19 @@ bool physical_memory_test_and_clear_dirty(ram_addr_t
> start,
> while (page < end) {
> unsigned long idx = page / DIRTY_MEMORY_BLOCK_SIZE;
> unsigned long offset = page % DIRTY_MEMORY_BLOCK_SIZE;
> - unsigned long num = MIN(end - page,
> - DIRTY_MEMORY_BLOCK_SIZE - offset);
>
> - dirty |= bitmap_test_and_clear_atomic(blocks->blocks[idx],
> - offset, num);
> - page += num;
> + if (bitmap_test_and_clear_atomic(blocks->blocks[idx], offset,
> 1)) {
> + if (dest) {
> + unsigned long k = page - (ramblock->offset >>
> TARGET_PAGE_BITS);
> + if (!test_and_set_bit(k, dest)) {
> + num_dirty++;
> + }
> + } else {
> + num_dirty++;
> + }
> + }
> +
> + page++;
> }
>
> mr_offset = (ram_addr_t)(start_page << TARGET_PAGE_BITS) -
> ramblock->offset;
> @@ -1131,18 +1150,18 @@ bool physical_memory_test_and_clear_dirty(ram_addr_t
> start,
> memory_region_clear_dirty_bitmap(ramblock->mr, mr_offset, mr_size);
> }
>
> - if (dirty) {
> + if (num_dirty) {
> physical_memory_dirty_bits_cleared(start, length);
> }
>
> - return dirty;
> + return num_dirty;
> }
>
> static void physical_memory_clear_dirty_range(ram_addr_t addr, ram_addr_t
> length)
> {
> - physical_memory_test_and_clear_dirty(addr, length,
> DIRTY_MEMORY_MIGRATION);
> - physical_memory_test_and_clear_dirty(addr, length, DIRTY_MEMORY_VGA);
> - physical_memory_test_and_clear_dirty(addr, length, DIRTY_MEMORY_CODE);
> + physical_memory_test_and_clear_dirty(addr, length,
> DIRTY_MEMORY_MIGRATION, NULL);
> + physical_memory_test_and_clear_dirty(addr, length, DIRTY_MEMORY_VGA,
> NULL);
> + physical_memory_test_and_clear_dirty(addr, length, DIRTY_MEMORY_CODE,
> NULL);
> }
>
> DirtyBitmapSnapshot *physical_memory_snapshot_and_clear_dirty
