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Date: Wed, 22 May 2019 19:19:45 +0300
From: Slawa Olhovchenkov <slw@zxy.spb.ru>
To: Alexander Motin <mav@FreeBSD.org>
Cc: lev@FreeBSD.org, Mark Johnston <markj@freebsd.org>,
 freebsd-fs@freebsd.org, freebsd-hackers@freebsd.org
Subject: Re: Commit r345200 (new ARC reclamation threads) looks suspicious to
 me - second potential problem
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On Wed, May 22, 2019 at 12:07:29PM -0400, Alexander Motin wrote:

> On 22.05.2019 11:50, Lev Serebryakov wrote:
> > On 22.05.2019 18:19, Alexander Motin wrote:
> > 
> >>>>  But looks like `arc_kmem_reap_soon()` is synchronous on FreeBSD! So,
> >>>> this `delay()` looks very wrong. Am I right?
> >>
> >> Why is it wrong?
> >  One second pause after synchronous operation to wait it completion?
> 
> No.  To rate-throttle them.  This gives UMA a second to get back into
> minimally steady state after we ripped all caches from it.  As I have
> told, we do not want to drain caches constantly in a tight loop, we want
> more or less steady state.

And also (posible) additionaly delay arc_get_data_impl().
This is incorrectly throttling implementation.