Date: Thu, 05 Nov 2015 12:25:38 -0800 From: Kirk McKusick <mckusick@mckusick.com> To: Konstantin Belousov <kostikbel@gmail.com> Cc: fs@freebsd.org Subject: Re: an easy (?) question on namecache sizing Message-ID: <201511052025.tA5KPcLF066724@chez.mckusick.com> In-Reply-To: <20151105195648.GK2257@kib.kiev.ua>
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> Date: Thu, 5 Nov 2015 21:56:48 +0200
> From: Konstantin Belousov <kostikbel@gmail.com>
> To: Kirk McKusick <mckusick@mckusick.com>
> Subject: Re: an easy (?) question on namecache sizing
> Cc: fs@freebsd.org
>
> On Thu, Nov 05, 2015 at 10:56:55AM -0800, Kirk McKusick wrote:
>>
>> I propose that we update wantfreevnodes in sysctl_update_desiredvnodes()
>> so that it tracks the change in desiredvnodes:
>>
>> Index: /sys/kern/vfs_subr.c
>> ===================================================================
>> --- /sys/kern/vfs_subr.c (revision 290387)
>> +++ /sys/kern/vfs_subr.c (working copy)
>> @@ -293,6 +293,7 @@
>> if (old_desiredvnodes != desiredvnodes) {
>> + wantfreevnodes = desiredvnodes / 4;
>> vfs_hash_changesize(desiredvnodes);
>> cache_changesize(desiredvnodes);
>> }
>> return (0);
>> }
>>
>> Otherwise bumping up desiredvnodes will be less effective than expected.
>>
>> I see that Bruce has also suggested this change in his more extensive
>> revisions.
>
> I think the idea is right, but the implementation is not. Just changing
> wantfreevnodes after desirevnodes was reduced, creates a window where an
> other thread could see small value for desiredvnodes, but large value
> for wantfreevnodes. Then, e.g. vlrureclaim() would go wild. IMO it should
> ensure that the observable values are non-contradictory.
Does moving the setting of wantfreevnodes before the cache size changes
(as redone above) close the window enough? The vlrureclaim() function
operates slowly enough that a brief period of inconsistency seems
unimportant. Changing desiredvnodes happens very rarely. And at the moment
we are not correcting wantfreevnodes at all. Or am I missing some key point?
Kirk McKusick
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