Date: Tue, 17 Aug 2021 03:19:35 +0200 From: Mateusz Guzik <mjguzik@gmail.com> To: Alan Somers <asomers@freebsd.org> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: sysctl is too slow Message-ID: <CAGudoHG%2BLjJQjxenNdrcfTLtnnkOr2jC-bpcX_BWtO-CSZTYAw@mail.gmail.com> In-Reply-To: <CAOtMX2jVzFURn6S0W9ygDpAjEK78ApEjz0C8hQYQG6UWPYY-Zw@mail.gmail.com> References: <CAOtMX2h7xkDM=GsPVyiWNcqxfRo7euZuuquSMn-y=PY5zRZNjg@mail.gmail.com> <CAGudoHGxWBLW2D6JX8mQCPwgM=ngt%2B3uZmwxK5p7yM6XeXXjsQ@mail.gmail.com> <CAOtMX2jVzFURn6S0W9ygDpAjEK78ApEjz0C8hQYQG6UWPYY-Zw@mail.gmail.com>
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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>
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