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

[Alexander Motin]

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`.

-- 
Alexander Motin