Date: Mon, 16 Aug 2021 19:38:56 -0600 From: Alan Somers <asomers@freebsd.org> To: Mateusz Guzik <mjguzik@gmail.com> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: sysctl is too slow Message-ID: <CAOtMX2iwRCwykfb=sumDGjWMRZ1HeRJGk2POBTDz12CjsihU1A@mail.gmail.com> In-Reply-To: <CAGudoHG%2BLjJQjxenNdrcfTLtnnkOr2jC-bpcX_BWtO-CSZTYAw@mail.gmail.com> References: <CAOtMX2h7xkDM=GsPVyiWNcqxfRo7euZuuquSMn-y=PY5zRZNjg@mail.gmail.com> <CAGudoHGxWBLW2D6JX8mQCPwgM=ngt%2B3uZmwxK5p7yM6XeXXjsQ@mail.gmail.com> <CAOtMX2jVzFURn6S0W9ygDpAjEK78ApEjz0C8hQYQG6UWPYY-Zw@mail.gmail.com> <CAGudoHG%2BLjJQjxenNdrcfTLtnnkOr2jC-bpcX_BWtO-CSZTYAw@mail.gmail.com>
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--0000000000003d160c05c9b762f8 Content-Type: text/plain; charset="UTF-8" Actually, I did get a flamegraph, and only 0.77% of samples were in ZFS. On Mon, Aug 16, 2021 at 7:19 PM Mateusz Guzik <mjguzik@gmail.com> wrote: > On 8/16/21, Alan Somers <asomers@freebsd.org> wrote: > > Yes, I see what you're talking about now. There are a bunch of linked > > lists in sysctl_find_oid etc. Good point. > > -Alan > > > > You still want to get a flamegraph, chances are most of the problem is in > zfs. > > > On Mon, Aug 16, 2021 at 1:30 PM Mateusz Guzik <mjguzik@gmail.com> wrote: > > > >> Last time I checked lookup of a sysctl was very bad with linear scans > all > >> over. > >> > >> Short of complete revamp of the entire thing I would start with > >> replacing the scans with a RB tree at each level. As is if you indeed > >> have 5000 datasets, you are doing increasingly longer walks. > >> > >> On 8/16/21, Alan Somers <asomers@freebsd.org> wrote: > >> > ztop feels very sluggish on a server with 5000 ZFS datasets. Dtrace > >> shows > >> > that almost all of its time is spent in sys_sysctl. ktrace shows that > >> both > >> > ztop and sysctl(8) call sys_sysctl a total of five times for each > >> > sysctl > >> > they care about: > >> > > >> > 1) To get the next oid > >> > 2) To get the sysctl's name > >> > 3) To get the oidfmt > >> > 4) To get the size of the value > >> > 5) To get the value itself. > >> > > >> > Each of these steps takes about equal time, and together all five take > >> > about 100us. If the time per call is mostly syscall overhead, then > the > >> > process could be sped up by 80% by combining all of these things into > a > >> > single syscall: return the next oid, its name, its format, the size of > >> its > >> > value, and optimistically the value itself, assuming the user passed a > >> > sufficiently large buffer. > >> > > >> > Am I missing something? Is there any other reason why sysctl is so > >> > slow? > >> > Or should I forget about it, and try to export ZFS's dataset stats > >> through > >> > devstat instead? > >> > -Alan > >> > > >> > >> > >> -- > >> Mateusz Guzik <mjguzik gmail.com> > >> > > > > > -- > Mateusz Guzik <mjguzik gmail.com> > --0000000000003d160c05c9b762f8--
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