Date: Thu, 30 Nov 2017 10:49:23 -0800 From: Larry McVoy <lm@mcvoy.com> To: Warner Losh <imp@bsdimp.com> Cc: Larry McVoy <lm@mcvoy.com>, "freebsd-arch@freebsd.org" <freebsd-arch@freebsd.org>, Scott Long <scottl@netflix.com>, Kevin Bowling <kbowling@llnw.com>, Drew Gallatin <gallatin@netflix.com> Subject: Re: small patch for pageout. Comments? Message-ID: <20171130184923.GA30262@mcvoy.com> In-Reply-To: <CANCZdfqL9ZsKTfFi%2BvsCTh3yaNjtwaYYY3fvivdbNybBnujawg@mail.gmail.com> References: <20171130173424.GA811@mcvoy.com> <CANCZdfqL9ZsKTfFi%2BvsCTh3yaNjtwaYYY3fvivdbNybBnujawg@mail.gmail.com>
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On Thu, Nov 30, 2017 at 11:37:35AM -0700, Warner Losh wrote: > On Thu, Nov 30, 2017 at 10:34 AM, Larry McVoy <lm@mcvoy.com> wrote: > > > In a recent numa meeting that Scott called, Jeff suggested a small > > patch to the pageout daemon (included below). > > > > It's rather dramatic the difference it makes for me. If I arrange to > > thrash the crap out of memory, without this patch the kernel is so > > borked with all the processes in disk wait that I can't kill them, > > I can't reboot, my only option is to power off. > > > > With the patch there is still some borkage, the kernel is randomly > > killing processes because of out of mem, it should kill one of my > > processes that is causing the problem but it doesn't, it killed > > random stuff like dhclient, getty (logged me out), etc. > > > > But the system is responsive. > > > > What the patch does is say "if we have more than one core, don't sleep > > in pageout, just keep running until we freed enough mem". > > > > Comments? > > > > Just to confirm why this patch works. > > For UP systems, we have to pause here to allow work to complete, otherwise > we can't switch to their threads to complete the I/Os. For MP, however, we > can continue to schedule more work because that work can be completed on > other CPUs. This parallelism greatly increases the pageout rate, allowing > the system to keep up better when some ass-hat process (or processes) is > thrashing memory. Yep. > I'm pretty sure the UP case was also designed to not flood the lower layers > with work, starving other consumers. Does this result in undo flooding, and > would we get better results if we could schedule up to the right amount of > work rather flooding in the MP case? I dunno if there is a "right amount". I could make it a little smarter by keeping track of how many pages we freed and sleep if we freed none in a scan (which seems really unlikely). All I know for sure is that without this you can lock up the system to the point it takes a power cycle to unwedge it. With this the system is responsive. Rather than worrying about the smartness, I'd argue this is an improvement, ship it, and then I can go look at how the system decides to kill processes (because that's currently busted).
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