Date: Wed, 28 Feb 2018 13:51:36 -0800 From: bob prohaska <fbsd@www.zefox.net> To: Ian Lepore <ian@freebsd.org> Cc: Mike <the.lists@mgm51.com>, freebsd-arm@freebsd.org, bob prohaska <fbsd@www.zefox.net> Subject: Re: Is maximum swap usage tunable? Message-ID: <20180228215136.GB29481@www.zefox.net> In-Reply-To: <1519848913.91697.387.camel@freebsd.org> References: <20180228170311.GA26187@www.zefox.net> <a759ecea-83f4-b0b2-7113-c39633f68637@mgm51.com> <20180228185517.GB26187@www.zefox.net> <1519848913.91697.387.camel@freebsd.org>
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On Wed, Feb 28, 2018 at 01:15:13PM -0700, Ian Lepore wrote: > On Wed, 2018-02-28 at 10:55 -0800, bob prohaska wrote: > > On Wed, Feb 28, 2018 at 12:20:56PM -0500, Mike wrote: > > > > > > On 2/28/2018 12:03 PM, bob prohaska wrote: > > > > > > > > In watching system compilations on an RPi3 it looks as if the > > > > system starts killing processes with "out of swap" warnings? > > > > well below 50% of full utilization (in this case, 2 GB). One > > > > recent instance of make -j4 kernel-toolchain killed llvm-tblgen > > > > with only 34% of the swap in use. > > > > > > > > Is the maximum swap usage limit adjustable in any way? I didn't > > > > recognize anything useful in the page at > > > > https://www.freebsd.org/cgi/man.cgi?query=sysctl(8)&sektion=&manpath=freebsd-release-ports? > > > > > > > > It's possible the problem is really swap speed, rather than size, so I'd > > > > like to try changing size limits if possible. The swap media claims 2-3? > > > > MB/sec random write speed and observations with gstat seem to support the > > > > claim, but transient stalls are hard to observe. An RPi2 with similar? > > > > hardware seems to have no problems. > > > > > > > > > > > > > > > It's possible the problem is really swap speed > > > I was running into swap speed / timeout issues.??There were messages on > > > the console to that effect. > > > > > > Once I put the swap space on rotating rust, that part of the compile > > > problem disappeared.??I use 1GB swap space. > > > > > The latest kernel versions seem to have largely done away with the > > "indefinite wait buffobj" warnings. They're few and far between, > > the compile proceeds unless they're abundant. The fact that armv7 > > has no problem, and the system is reporting "out of swap" with 34% > > in use strikes me as suspicious. Kernel and userland are in sync,? > > so the figure of 34% swap usage is probably accurate.? > > > > Perhaps my question is better phrased thus: How does FreeBSD-arm? > > determine when it's out of swap? > > > > Thanks for reading, > > > > bob prohaska > > You are reading too much into the phrase "out of swap". It doesn't > mean literally "all sectors of all swapfiles are occupied with live > data". ?It just means "out of memory". ?It can happen because it's > unable to move more data to swap fast enough due to some short peak > demand. ?It can happen for other reasons too, like all disk buffers in > use, or running out of resources to track the data in swap, or the > system can't find any more pages eligible to be moved out to swap. > > It looks like the message that indicates swap space is truly full is > > ? swap_pager: out of swap space > > Is that what you're seeing? ?Or are you seeing processes killed with an > out of swap space message? > I'm seeing Feb 27 18:41:50 www kernel: pid 46399 (llvm-tblgen), uid 0, was killed: out of swap space but never anything referencing swap_pager. As it happens, the machine is still using two swap partitions, one on USB flash and one on the microSD card. Another correspondent reported that swap on microSD was troublesome, I thought that problem was fixed. Maybe I was mistaken and should repeat the experiment. Thanks for clarifying the terminology! bob prohaska
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