svn commit: r348117 - head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs

[Slawa Olhovchenkov]

On Wed, May 22, 2019 at 04:00:58PM -0400, Alexander Motin wrote:

> On 22.05.2019 15:49, Slawa Olhovchenkov wrote:
> > On Wed, May 22, 2019 at 06:43:48PM +0000, Alexander Motin wrote:
> >> Author: mav
> >> Date: Wed May 22 18:43:48 2019
> >> New Revision: 348117
> >> URL: https://svnweb.freebsd.org/changeset/base/348117
> >>
> >> Log:
> >>   Allocate buffers smaller then ABD chunk size as linear.
> >>   
> >>   This allows to reduce memory waste by letting UMA to put multiple small
> >>   buffers into one memory page slab.  The page sharing means that UMA
> >>   may not be able to free memory page when some of buffers are freed, but
> >>   alternatively memory used by that buffer would just be wasted from the
> >>   beginning.
> >>   
> >>   This change follows alike change in ZoL, but unlike Linux (according to
> >>   my understanding of it from comments) FreeBSD never shares slabs bigger
> >>   then one memory page, so this should be even less invasive then there.
> >>   
> >>   MFC after:	2 weeks
> >>   Sponsored by:	iXsystems, Inc.
> >>
> >> Modified:
> >>   head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/abd.c
> >>
> >> Modified: head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/abd.c
> >> ==============================================================================
> >> --- head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/abd.c	Wed May 22 17:42:22 2019	(r348116)
> >> +++ head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/abd.c	Wed May 22 18:43:48 2019	(r348117)
> >> @@ -290,7 +290,7 @@ abd_free_struct(abd_t *abd)
> >>  abd_t *
> >>  abd_alloc(size_t size, boolean_t is_metadata)
> >>  {
> >> -	if (!zfs_abd_scatter_enabled)
> >> +	if (!zfs_abd_scatter_enabled || size <= zfs_abd_chunk_size)
> >>  		return (abd_alloc_linear(size, is_metadata));
> > 
> > may be `size < zfs_abd_chunk_size`?
> 
> Why?  It should be the same from memory usage, but I see theoretical
> benefits from having linear buffer with one page rather then scatter
> list with one page.  Plus ZoL also had there `size <= PAGESIZE`.

May be malloc() for page size more effictive? Not sure.
Less overhead for kegs and etc.