Hi

Current migration code recalculates the amount of RAM each time that
is needed.  This calculation requires RCU and other operations.
During migration we disable hot/unplug of memory, so we can store it.

Notice the times difference, and specially that ram_bytes_total()
don't appears anymore in the perf output.

total time: 75852 ms
downtime: 264 ms
setup: 273 ms
transferred ram: 19671939 kbytes
throughput: 2132.28 mbps
remaining ram: 0 kbytes
total ram: 1077936904 kbytes
duplicate: 265170289 pages
skipped: 0 pages
normal: 4316628 pages
normal bytes: 17266512 kbytes
dirty sync count: 4
page size: 4 kbytes
multifd bytes: 17341329 kbytes
pages-per-second: 1236658
precopy ram: 2330608 kbytes
downtime ram: 1 kbytes

  37.97%  live_migration   qemu-system-x86_64       [.] buffer_zero_avx512
  10.42%  live_migration   qemu-system-x86_64       [.] ram_find_and_save_block.part.0
   6.67%  live_migration   qemu-system-x86_64       [.] add_to_iovec
   3.71%  live_migration   qemu-system-x86_64       [.] ram_bytes_total_common
   2.79%  live_migration   qemu-system-x86_64       [.] qemu_ram_is_migratable
   2.69%  live_migration   qemu-system-x86_64       [.] qemu_put_byte.part.0
   2.41%  live_migration   qemu-system-x86_64       [.] bitmap_test_and_clear_atomic
   1.55%  live_migration   qemu-system-x86_64       [.] qemu_put_be32
   1.26%  live_migration   qemu-system-x86_64       [.] find_next_bit
   1.07%  multifdsend_0    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   1.07%  multifdsend_13   [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   1.06%  multifdsend_6    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   1.05%  multifdsend_2    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   1.04%  multifdsend_15   [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   1.03%  multifdsend_12   [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   1.02%  live_migration   qemu-system-x86_64       [.] migrate_ignore_shared
   1.01%  multifdsend_7    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   1.01%  multifdsend_3    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   1.01%  multifdsend_10   [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.98%  live_migration   qemu-system-x86_64       [.] ram_save_iterate
   0.96%  multifdsend_4    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.93%  multifdsend_8    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.92%  multifdsend_5    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.90%  multifdsend_14   [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.88%  multifdsend_9    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.85%  multifdsend_1    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.83%  multifdsend_11   [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.61%  live_migration   qemu-system-x86_64       [.] save_zero_page_to_file.part.0
   0.48%  live_migration   [kernel.kallsyms]        [k] copy_user_enhanced_fast_string

Migration status: completed
total time: 70033 ms
downtime: 279 ms
setup: 280 ms
transferred ram: 19692747 kbytes
throughput: 2312.82 mbps
remaining ram: 0 kbytes
total ram: 1077936904 kbytes
duplicate: 265164421 pages
skipped: 0 pages
normal: 4322415 pages
normal bytes: 17289660 kbytes
dirty sync count: 3
page size: 4 kbytes
multifd bytes: 17362190 kbytes
pages-per-second: 2523447
precopy ram: 2330555 kbytes
downtime ram: 1 kbytes

  43.64%  live_migration   qemu-system-x86_64       [.] buffer_zero_avx512
  11.32%  live_migration   qemu-system-x86_64       [.] ram_find_and_save_block.part.0
   7.60%  live_migration   qemu-system-x86_64       [.] add_to_iovec
   2.95%  live_migration   qemu-system-x86_64       [.] qemu_put_byte.part.0
   2.73%  live_migration   qemu-system-x86_64       [.] bitmap_test_and_clear_atomic
   1.76%  live_migration   qemu-system-x86_64       [.] qemu_put_be32
   1.44%  live_migration   qemu-system-x86_64       [.] find_next_bit
   0.84%  multifdsend_1    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.84%  multifdsend_7    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.81%  multifdsend_15   [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.80%  multifdsend_4    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.80%  multifdsend_3    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.79%  multifdsend_12   [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.79%  multifdsend_14   [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.79%  multifdsend_11   [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.78%  multifdsend_13   [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.78%  live_migration   qemu-system-x86_64       [.] ram_save_iterate
   0.77%  multifdsend_9    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.77%  multifdsend_5    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.77%  multifdsend_10   [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.77%  multifdsend_2    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.77%  multifdsend_6    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.76%  multifdsend_8    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.71%  multifdsend_0    [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.66%  live_migration   qemu-system-x86_64       [.] save_zero_page_to_file.part.0
   0.62%  live_migration   qemu-system-x86_64       [.] qemu_ram_is_migratable
   0.54%  live_migration   [kernel.kallsyms]        [k] copy_user_enhanced_fast_string
   0.51%  live_migration   qemu-system-x86_64       [.] qemu_put_byte

Please, review.

Thanks, Juan.

Juan Quintela (2):
  migration: Split ram_bytes_total_common() in two functions
  migration: Calculate ram size once

 migration/ram.c | 31 ++++++++++++++++++-------------
 1 file changed, 18 insertions(+), 13 deletions(-)

-- 
2.35.1