Re: [Qemu-devel] [PATCHv5 00/10] buffer_is_zero / migration optimizations

[Juan Quintela <quintela@redhat.com>]

Peter Lieven <pl@kamp.de> wrote:
> this is v5 of my patch series with various optimizations in
> zero buffer checking and migration tweaks.
>
> thanks especially to Eric Blake, Orit Wassermann and Paolo Bonzini
> for reviewing.
>
> v5:
> - move zero splat vector to a different patch
> - fix indentation of can_user_buffer_find_nonzero_offset()
> - do not unroll the first loop in buffer_find_nonzero_offset()
>   to optimize it for zero page checking
> - use an older unrolled version of find_next_bit() without
>   SIMD instruction as there is no evidence that the vectorized
>   version is better if not even worse and the code is easier
>   to understand.
> - added a word in the commit message of patch 8 
>   about the skipped pages field in QMP MigrationStats.
> - fixed the order of key-value pairs of MigrationStats in
>   qapi-schema.json
> - updated info about the performance benefit of is_zero_page()
>   to the latest benchmark results in the commit message.
>
> v4:
> - do not inline buffer_find_nonzero_offset()
> - inline can_usebuffer_find_nonzero_offset() correctly
> - readd asserts in buffer_find_nonzero_offset() as profiling
>   shows they do not hurt.
> - change last occurences of scalar 8 by 
>   BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR
> - avoid deferencing p already in patch 5 where we
>   know that the page (p) is zero
> - explicitly set bytes_sent = 0 if we skip a zero page.
>   bytes_sent was 0 before, but it was not obvious.
> - add accounting information for skipped zero pages
> - fix errors reported by checkpatch.pl
>
> v3:
> - remove asserts, inline functions and add a check
>   function if buffer_find_nonzero_offset() can be used.
> - use above check function in buffer_is_zero() and
>   find_next_bit().
> - use buffer_is_nonzero_offset() directly to find
>   zero pages. we know that all requirements are met
>   for memory pages.
> - fix C89 violation in buffer_is_zero().
> - avoid derefencing p in ram_save_block() if we already
>   know the page is zero.
> - fix initialization of last_offset in reset_ram_globals().
> - avoid skipping pages with offset == 0 in bulk stage in
>   migration_bitmap_find_and_reset_dirty().
> - compared to v1 check for zero pages also after bulk
>   ram migration as there are guests (e.g. Windows) which
>   zero out large amount of memory while running.
>
> v2:
> - fix description, add trivial zero check and add asserts 
>   to buffer_find_nonzero_offset.
> - add a constant for the unroll factor of buffer_find_nonzero_offset
> - replace is_dup_page() by buffer_is_zero()
> - added test results to xbzrle patch
> - optimize descriptions
>
> Peter Lieven (10):
>   move vector definitions to qemu-common.h
>   add a zero splat vector to qemu-common.h
>   cutils: add a function to find non-zero content in a buffer
>   buffer_is_zero: use vector optimizations if possible
>   bitops: unroll while loop in find_next_bit()
>   migration: search for zero instead of dup pages
>   migration: add an indicator for bulk state of ram migration
>   migration: do not sent zero pages in bulk stage
>   migration: do not search dirty pages in bulk stage
>   migration: use XBZRLE only after bulk stage

Really nice series.
I did some minor review changes, but they are more nits that anynthing
else.

Thanks, Juan.

Reviewed-by: Juan Quintela <quintela@redhat.com>