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Date:      Wed, 25 Feb 2015 17:18:10 -0700
From:      Warner Losh <imp@bsdimp.com>
To:        John Baldwin <jhb@FreeBSD.org>
Cc:        Konstantin Belousov <kostikbel@gmail.com>, freebsd-arch@freebsd.org
Subject:   Re: locks and kernel randomness...
Message-ID:  <871FF097-7372-4901-8202-003398B6ABE3@bsdimp.com>
In-Reply-To: <1774232.7ZAkabLA24@ralph.baldwin.cx>
References:  <DD06E2EA-68D6-43D7-AA17-FB230750E55A@bsdimp.com> <54ED87E9.8030706@astrodoggroup.com> <20150225085659.GA74514@kib.kiev.ua> <1774232.7ZAkabLA24@ralph.baldwin.cx>

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> On Feb 25, 2015, at 1:20 PM, John Baldwin <jhb@FreeBSD.org> wrote:
>=20
> On Wednesday, February 25, 2015 10:56:59 AM Konstantin Belousov wrote:
>> The cost of the proposed patch, of course, is not the several
>> thousands of instructions in the rebalance. The problem with it is =
the
>> introduction of the new spinlock, which will be used in many places
>> after the introduction. The cost of the new and often used spinlock =
is
>> the increase of both interrupt latency and interrupt handler jitter =
and
>> cpu switch jitter.
>>=20
>> So neither buildworld timing, nor network throughput are adequate
>> to estimate the change.  It is system unresponsivness and loss of
>> the realtime behaviour up to some degree.
>>=20
>> I thought that it was obvious, at least after spinlocks were =
mentioned,
>> but apparently it is not, since proposals to measure the patch effect
>> by benchmarking buildworld or passing the traffic are made.
>=20
> +1
>=20
> The only thing I will add is that in general this makes the system =
more
> fragile and complex as well.  Please just stay with a regular mutex =
and change
> the scheduler to not use random() (which seems to be in progress?).  =
I'm not
> sure why we needed the extra 40 messages in this thread after that was
> effectively said the first time.

If you=E2=80=99d like to be helpful, please review =
https://reviews.freebsd.org/D1968
Harrison Grundy wrote it, and I put it in phabricator for him while he =
sorts
out his own account.

In a fit of passive aggressiveness, I=E2=80=99ve added everybody who =
commented on
this thread as reviewers, except Bruce and Harrison who don=E2=80=99t =
have accounts
yet.

Warner


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