Date: Fri, 16 Oct 2020 13:24:13 -0700 From: Doug Hardie <bc979@lafn.org> To: FreeBSD Mailing List <freebsd-questions@freebsd.org> Subject: Re: FreeBSD using swap even though there's a lot of free memory Message-ID: <89B99A9E-C3A2-47BE-8A0E-D296846946CB@mail.sermon-archive.info> In-Reply-To: <20201016195546.a3392f971837aa2eeace1325@sohara.org> References: <5f885b772d622_95aa2adab2b9c5b41576495c3@sirportly-app-02.mail> <CAHu1Y72pNNXt-i552F2JT%2ButhqAMjQdXLoRNofw6Xr0iwenjVw@mail.gmail.com> <20201016195546.a3392f971837aa2eeace1325@sohara.org>
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> On 16 October 2020, at 11:55, Steve O'Hara-Smith <steve@sohara.org> wrote: > > On Fri, 16 Oct 2020 11:35:42 -0700 > Michael Sierchio <kudzu@tenebras.com> wrote: > >> On Thu, Oct 15, 2020 at 7:24 AM Twingly Customer Support >> <team@twingly.com> wrote: >> >>> Hi, >>> >>> We have a server running FreeBSD 12.1-RELEASE-p10. We currently have a >>> problem where FreeBSD starting to swap when running ZFS scrub, even >>> though we have ~70G of free memory. >> >> >> tl;dr – this is normal, nothing to see, move along. > > Not when it starts running out of swap space while still reporting > 70G of free memory. I encountered this issue a year or so ago. In my case it turned out to be a process that was allocating anonymous segments using mmap. It tended to forget to free them after the usage was complete. As a result, the system would run out of swap even though there was plenty of free memory. Anonymous segments are mapped to swap space. I traced the problem using: procstat -va | grep df That generated a lot of data. I looked for processes that had an increasing number of df files. Eventually I found the right one and fixed it. Interesting observation: I have used FreeBSD since version 2.5 and for years it always had a small number of processes swapped out within an hour of a boot. Typically they were gettys and similar processes that never got used. However, with 12.x, I now find that swap info is always showing 0 blocks used even after weeks of operation. Something has changed for the better. It is also possible that greping for sw rather than df might give some useful information. I can't tell as none of my systems are using any swap. Actually, one system did not have any swap for over a year. I didn't notice that there was an issue mounting its swap. There were no performance indicators that issue existed. Nothing changed after I corrected the problem and added the swap. -- Doug
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