Date: Fri, 26 Jun 2020 11:13:44 -0400 From: Paul Mather <paul@gromit.dlib.vt.edu> To: =?utf-8?Q?Stefan_E=C3=9Fer?= <se@freebsd.org> Cc: freebsd-stable <freebsd-stable@freebsd.org>, Peter Jeremy <peter@rulingia.com> Subject: Re: swap space issues Message-ID: <FB7DCF2D-DB4A-4539-8A39-AB877C2517AD@gromit.dlib.vt.edu> In-Reply-To: <52753cf4-57db-93d9-d217-c8f812d6bc7c@freebsd.org> References: <CAEC7391qs%2BA-jMpR1RyvR-BmnLyiksXHkQUjsGeePuEZJfMciw@mail.gmail.com> <20200625000410.GA10210@eureka.lemis.com> <CAEC7390VDxbYSY%2B4_fEaYxwdSPzbFWUVTdHw=vbAgq%2Bnmv09Vw@mail.gmail.com> <20200625025248.GB10210@eureka.lemis.com> <CAEC73938Wjb5MHvLW36PdoAy_nso-tSN51AhUYydC6qxY99pog@mail.gmail.com> <E8763B97-2DB7-4C77-864D-08155168E352@gromit.dlib.vt.edu> <CAEC7391AHKXd0KfJdUGKMv1QRh_AtA1BrtqaQwy3dXEoJEMoDw@mail.gmail.com> <20200626102331.GA6406@server.rulingia.com> <52753cf4-57db-93d9-d217-c8f812d6bc7c@freebsd.org>
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On Jun 26, 2020, at 6:58 AM, Stefan E=C3=9Fer <se@freebsd.org> wrote: > Am 26.06.20 um 12:23 schrieb Peter Jeremy: >> On 2020-Jun-25 11:30:31 -0700, Donald Wilde <dwilde1@gmail.com> = wrote: >>> Here's 'pstat -s' on the i3 (which registers as cpu HAMMER): >>>=20 >>> Device 1K-blocks Used Avail Capacity >>> /dev/ada0s1b 33554432 0 33554432 0% >>> /dev/ada0s1d 33554432 0 33554432 0% >>> Total 67108864 0 67108864 0% >>=20 >> I strongly suggest you don't have more than one swap device on = spinning >> rust - the VM system will stripe I/O across the available devices and >> that will give particularly poor results when it has to seek between = the >> partitions. >=20 [[...]] >> As a further piece of arcana, vm.pageout_oom_seq is a count that = controls >> the number of passes before the pageout daemon gives up and starts = killing >> processes when it can't free up enough RAM. "out of swap space" = messages >> generally mean that this number is too low, rather than there being a >> shortage of swap - particularly if your swap device is rather slow. >=20 > I'm not sure that this specific sysctl is documented in such a way > that it is easy to find by people suffering from out-of-memory kills. >=20 > Perhaps it could be mentioned as a parameter that may need tuning in > the OOM message? >=20 > And while it does not come up that often in the mail list, it might > be better for many kinds of application if the default was increased > (a longer wait for resources might be more acceptable than the loss > of all results of a long running computation). The OOM issue is more pressing on platforms like FreeBSD/arm that tend = to have low RAM and slow writable storage such as SD card. There have = been several threads on the issues this creates (e.g., = https://docs.freebsd.org/cgi/getmsg.cgi?fetch=3D228789+0+archive/2018/free= bsd-arm/20180819.freebsd-arm = <https://docs.freebsd.org/cgi/getmsg.cgi?fetch=3D228789+0+archive/2018/fre= ebsd-arm/20180819.freebsd-arm>) that have led to some insight into how = to tune the OOM killer. One thing that becomes clear is that the "Out = of swap space" error message is misleading as often it really means = "Couldn't obtain RAM in a timely fashion." On hardware such as the = Raspberry Pi, it's often the case that the system has enough swap space: = it's just that it can't write to swap on SD card before the default = vm.pageout_oom_seq passes are exhausted and so the OOM killer starts = reaping active processes (like the clang trying to build clang:), and = all sorts of things start to break. :-) Cheers, Paul.=
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