svn commit: r348117 - head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs
Alexander Motin
mav at FreeBSD.org
Wed May 22 20:01:01 UTC 2019
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
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