Date: Mon, 22 Apr 2024 16:05:15 -0600 From: Alan Somers <asomers@freebsd.org> To: Karl Denninger <karl@denninger.net> Cc: freebsd-hackers@freebsd.org Subject: Re: Stressing malloc(9) Message-ID: <CAOtMX2irXo_hvrhQhw0eLjCBiH7hZMTR9notBn9aDEMTynQiuQ@mail.gmail.com> In-Reply-To: <b1e56d20-dc98-4fff-adec-3f8cfae26c05@denninger.net> References: <CAOtMX2jeDHS15bGgzD89AOAd1SzS_=FikorkCdv9-eAxCZ2P5w@mail.gmail.com> <ZiPaFw0q17RGE7cS@nuc> <CAOtMX2jk6%2BSvqMP7Cbmdk0KQCFZ34yWuir7n_8ewZYJF2MwPSg@mail.gmail.com> <ZiU6IZ29syVsg61p@nuc> <CAOtMX2j=yaYeE%2B-fycg2mRRC_Jb9p74cn_dcenhH2xRRxz1shg@mail.gmail.com> <CAOtMX2hDfX-T90x9Fb2Wh%2BvgLvw9fUGmaDxh-FWaYwBTPwFY6Q@mail.gmail.com> <b1e56d20-dc98-4fff-adec-3f8cfae26c05@denninger.net>
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On Mon, Apr 22, 2024 at 2:07=E2=80=AFPM Karl Denninger <karl@denninger.net>= wrote: > > On 4/22/2024 12:46, Alan Somers wrote: > > When I said "33kiB" I meant "33 pages", or 132 kB. And the solution > turns out to be very easy. Since I'm using ZFS on top of geli, with > the default recsize of 128kB, I'll just set > vfs.zfs.vdev.aggregation_limit to 128 kB. That way geli will never > need to allocate more than 128kB contiguously. ZFS doesn't even need > those big allocations to be contiguous; it's just aggregating smaller > operations to reduce disk IOPs. But aggregating up to 1MB (the > default) is overkill; any rotating HDD should easily be able to max > out its consecutive write IOPs with 128kB operation size. I'll add a > read-only sysctl for g_eli_alloc_sz too. Thanks Mark. > > -Alan > > Setting this on one of my production machines that uses zfs behind geli d= rops the load average quite materially with zero impact on throughput that = I can see (thus far.) I will run this for a while but it certainly doesn't= appear to have any negatives associated with it and does appear to improve= efficiency quite a bit. Great news! Also, FTR I should add that this advice only applies to people who use HDDs. For SSDs zfs uses a different aggregation limit, and the default value is already low enough. -Alan
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