Re: sysctl is too slow

From: Mateusz Guzik <mjguzik_at_gmail.com>
Date: Tue, 17 Aug 2021 03:19:35 +0200
On 8/16/21, Alan Somers <asomers_at_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_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>
>>
>


-- 
Mateusz Guzik <mjguzik gmail.com>
Received on Tue Aug 17 2021 - 01:19:35 UTC

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