On 13/12/2017 20:52, Peter Jeremy wrote:
> On 2017-Dec-06 10:01:02 +0000, Baptiste Daroussin <[email protected]> wrote:
>> Author: bapt
>> Date: Wed Dec 6 10:01:02 2017
>> New Revision: 326619
>> URL: https://svnweb.freebsd.org/changeset/base/326619
>>
>> Log:
>> MFC r325851:
>>
>> remove the poor emulation of the IllumOS needfree global variable to prevent
>> the ARC reclaim thread running longer than needed.
>>
>
> This change appears to break the backpressure mechanism that forces
> ARC to release wired memory. Running FreeBSD/amd64 r326743, I found
> that my system was thrashing badly with almost all memory (~31GB out
> of 32GB) wired (including 19GB ARC, 7GB VBox guests). Running a tool
> that just allocates and dirties a large amount of memory had no effect
> on the ARC size. Reverting this change only restores normal behaviour
> where ARC will shrink under pressure.
>
> My ZFS boot tunables are:
> vfs.zfs.arc_max="24G"
> vfs.zfs.arc_meta_limit="12G"
>
Peter,
could you please try to monitor 'arc-needfree' DTrace probe during periods when
you are trying to put a pressure on the memory?
The probe's argument (signed) tells the ARC's idea of the memory availability /
shortage.
bapt wrote:
>> @@ -6310,9 +6294,7 @@ arc_lowmem(void *arg __unused, int howto __unused)
>> {
>>
>> mutex_enter(&arc_reclaim_lock);
>> - /* XXX: Memory deficit should be passed as argument. */
>> - needfree = btoc(arc_c >> arc_shrink_shift);
>> - DTRACE_PROBE(arc__needfree);
>> + DTRACE_PROBE1(arc__needfree, int64_t, ((int64_t)freemem -
>> zfs_arc_free_target) * PAGESIZE);
>> cv_signal(&arc_reclaim_thread_cv);
>>
>> /*
--
Andriy Gapon
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