Re: sysctl is too slow
- Reply: Alan Somers : "Re: sysctl is too slow"
- In reply to: Alan Somers : "Re: sysctl is too slow"
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Date: Mon, 23 Aug 2021 12:04:49 UTC
So is this something you plan on fixing? On 8/17/21, Alan Somers <asomers@freebsd.org> wrote: > 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> >> > -- Mateusz Guzik <mjguzik gmail.com>