Hi Peter,
Sorry for the delay as i was busy on other things.
On 06/19/2018 03:30 PM, Peter Xu wrote:
On Mon, Jun 04, 2018 at 05:55:14PM +0800, guangrong.xiao@xxxxxxxxx wrote:
From: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxx>
Detecting zero page is not a light work, we can disable it
for compression that can handle all zero data very well
Is there any number shows how the compression algo performs better
than the zero-detect algo? Asked since AFAIU buffer_is_zero() might
be fast, depending on how init_accel() is done in util/bufferiszero.c.
This is the comparison between zero-detection and compression (the target
buffer is all zero bit):
Zero 810 ns Compression: 26905 ns.
Zero 417 ns Compression: 8022 ns.
Zero 408 ns Compression: 7189 ns.
Zero 400 ns Compression: 7255 ns.
Zero 412 ns Compression: 7016 ns.
Zero 411 ns Compression: 7035 ns.
Zero 413 ns Compression: 6994 ns.
Zero 399 ns Compression: 7024 ns.
Zero 416 ns Compression: 7053 ns.
Zero 405 ns Compression: 7041 ns.
Indeed, zero-detection is faster than compression.
However during our profiling for the live_migration thread (after reverted this patch),
we noticed zero-detection cost lots of CPU:
12.01% kqemu qemu-system-x86_64 [.] buffer_zero_sse2 ◆
7.60% kqemu qemu-system-x86_64 [.] ram_bytes_total ▒
6.56% kqemu qemu-system-x86_64 [.] qemu_event_set ▒
5.61% kqemu qemu-system-x86_64 [.] qemu_put_qemu_file ▒
5.00% kqemu qemu-system-x86_64 [.] __ring_put ▒
4.89% kqemu [kernel.kallsyms] [k] copy_user_enhanced_fast_string ▒
4.71% kqemu qemu-system-x86_64 [.] compress_thread_data_done ▒
3.63% kqemu qemu-system-x86_64 [.] ring_is_full ▒
2.89% kqemu qemu-system-x86_64 [.] __ring_is_full ▒
2.68% kqemu qemu-system-x86_64 [.] threads_submit_request_prepare ▒
2.60% kqemu qemu-system-x86_64 [.] ring_mp_get ▒
2.25% kqemu qemu-system-x86_64 [.] ring_get ▒
1.96% kqemu libc-2.12.so [.] memcpy
After this patch, the workload is moved to the worker thread, is it
acceptable?
From compression rate POV of course zero page algo wins since it
contains no data (but only a flag).
Yes it is. The compressed zero page is 45 bytes that is small enough i think.
Hmm, if you do not like, how about move detecting zero page to the work thread?
Thanks!
