Re: sysctl is too slow

From: Alan Somers <asomers_at_freebsd.org>
Date: Mon, 16 Aug 2021 15:13:11 -0600
Yes, I see what you're talking about now.  There are a bunch of linked
lists in sysctl_find_oid etc.  Good point.
-Alan

On Mon, Aug 16, 2021 at 1:30 PM Mateusz Guzik <mjguzik_at_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_at_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>
>
Received on Mon Aug 16 2021 - 21:13:11 UTC

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