Date: Mon, 18 Mar 2019 09:14:40 -0500 From: Karl Denninger <karl@denninger.net> To: freebsd-stable@freebsd.org Subject: Re: Observations from a ZFS reorganization on 12-STABLE Message-ID: <76f222db-8b75-80ef-ce48-a43217f10e60@denninger.net> In-Reply-To: <20190318091431.W52549@mulder.mintsol.com> References: <58eb1994-41bd-cd22-be66-0024bcbc36e6@denninger.net> <e5c648b4-ba73-5d3a-2924-be4d52e4c267@grosbein.net> <2baf16fd-3767-1dda-d519-995f7ebaf0cb@ingresso.co.uk> <20190318091431.W52549@mulder.mintsol.com>
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This is a cryptographically signed message in MIME format. --------------ms060507040309090209010401 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable On 3/18/2019 08:37, Walter Cramer wrote: > I suggest caution in raising vm.v_free_min, at least on 11.2-RELEASE > systems with less RAM.=C2=A0 I tried "65536" (256MB) on a 4GB mini-serv= er, > with vfs.zfs.arc_max of 2.5GB.=C2=A0 Bad things happened when the cron > daemon merely tried to run `periodic daily`. > > A few more details - ARC was mostly full, and "bad things" was 1: > `pagedaemon` seemed to be thrashing memory - using 100% of CPU, with > little disk activity, and 2: many normal processes seemed unable to > run. The latter is probably explained by `man 3 sysctl` (see entry for > "VM_V_FREE_MIN"). > > > On Mon, 18 Mar 2019, 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. >> Raising free_target should *not* result in that sort of thrashing.=C2=A0 However, that's not really a fix standing alone either since the underlying problem is not being addressed by either change.=C2=A0 It is especially dangerous to raise the pager wakeup thresholds if you still run into UMA allocated-but-not-in-use not being cleared out issues as there's a risk of severe pathological behavior arising that's worse than the original problem. 11.1 and before (I didn't have enough operational experience with 11.2 to know, as I went to 12.x from mostly-11.1 installs around here) were essentially unusable in my workload without either my patch set or the Phabricator one. This is *very* workload-specific however, or nobody would use ZFS on earlier releases, and many do without significant problems. --=20 Karl Denninger karl@denninger.net <mailto:karl@denninger.net> /The Market Ticker/ /[S/MIME encrypted email preferred]/ --------------ms060507040309090209010401 Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename="smime.p7s" Content-Description: S/MIME Cryptographic Signature MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgMFADCABgkqhkiG9w0BBwEAAKCC DdgwggagMIIEiKADAgECAhMA5EiKghDOXrvfxYxjITXYDdhIMA0GCSqGSIb3DQEBCwUAMIGL MQswCQYDVQQGEwJVUzEQMA4GA1UECAwHRmxvcmlkYTESMBAGA1UEBwwJTmljZXZpbGxlMRkw FwYDVQQKDBBDdWRhIFN5c3RlbXMgTExDMRgwFgYDVQQLDA9DdWRhIFN5c3RlbXMgQ0ExITAf BgNVBAMMGEN1ZGEgU3lzdGVtcyBMTEMgMjAxNyBDQTAeFw0xNzA4MTcxNjQyMTdaFw0yNzA4 MTUxNjQyMTdaMHsxCzAJBgNVBAYTAlVTMRAwDgYDVQQIDAdGbG9yaWRhMRkwFwYDVQQKDBBD 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