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Date:      Mon, 18 Mar 2019 08:40:52 -0500
From:      Karl Denninger <karl@denninger.net>
To:        freebsd-stable@freebsd.org
Subject:   Re: Observations from a ZFS reorganization on 12-STABLE
Message-ID:  <6e9a7a55-3eec-f1b4-61cc-45da4a22eb85@denninger.net>
In-Reply-To: <2baf16fd-3767-1dda-d519-995f7ebaf0cb@ingresso.co.uk>
References:  <58eb1994-41bd-cd22-be66-0024bcbc36e6@denninger.net> <e5c648b4-ba73-5d3a-2924-be4d52e4c267@grosbein.net> <2baf16fd-3767-1dda-d519-995f7ebaf0cb@ingresso.co.uk>
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[-- Attachment #1 --]

On 3/18/2019 08:07, Pete French wrote:
>
>
> On 17/03/2019 21:57, Eugene Grosbein wrote:
>> I agree. Recently I've found kind-of-workaround for this problem:
>> increase vm.v_free_min so when "FREE" memory goes low,
>> page daemon wakes earlier and shrinks UMA (and ZFS ARC too) moving
>> some memory
>> from WIRED to FREE quick enough so it can be re-used before bad
>> things happen.
>>
>> But avoid increasing vm.v_free_min too much (e.g. over 1/4 of total RAM)
>> because kernel may start behaving strange. For 16Gb system it should
>> be enough
>> to raise vm.v_free_min upto 262144 (1GB) or 131072 (512M).
>>
>> This is not permanent solution in any way but it really helps.
>
> Ah, thats very interesting, thankyou for that! I;ve been bitten by
> this issue too in the past, and it is (as mentioned) much improved on
> 12, but the act it could still cause issues worries me.
>
>
The code patch I developed originally essentially sought to have the ARC
code pare back before the pager started evicting working set.  A second
crack went after clearing allocated-but-not-in-use UMA.

v_free_min may not be the right place to do this -- see if bumping up
vm.v_free_target also works.

I'll stick this on my "to do" list, but it's much less critical in my
applications than it was with 10.x and 11.x, both of which suffered from
it much more-severely to the point that I was getting "stalls" that in
some cases went on for 10 or more seconds due to things like your shell
being evicted to swap to make room for arc, which is flat-out nuts. 
That, at least, doesn't appear to be a problem with 12.

-- 
Karl Denninger
karl@denninger.net <mailto:karl@denninger.net>
/The Market Ticker/
/[S/MIME encrypted email preferred]/

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