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From:   John Ogness <john.ogness@linutronix.de>
To:     lijiang <lijiang@redhat.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Petr Mladek <pmladek@suse.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrea Parri <parri.andrea@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        kexec@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/2] printk: replace ringbuffer
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        <dc4ca9b5-d2a2-03af-c186-204a3aad2399@redhat.com>
        <20200205044848.GH41358@google.com>
        <20200205050204.GI41358@google.com>
        <88827ae2-7af5-347b-29fb-cffb94350f8f@redhat.com>
        <20200205063640.GJ41358@google.com> <877e11h0ir.fsf@linutronix.de>
        <cd7509a5-48fd-e652-90f4-1e0fe2311134@redhat.com>
Date:   Wed, 05 Feb 2020 17:12:12 +0100
In-Reply-To: <cd7509a5-48fd-e652-90f4-1e0fe2311134@redhat.com>
        (lijiang@redhat.com's message of "Wed, 5 Feb 2020 18:19:02 +0800")
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On 2020-02-05, lijiang <lijiang@redhat.com> wrote:
> Do you have any suggestions about the size of CONFIG_LOG_* and
> CONFIG_PRINTK_* options by default?

The new printk implementation consumes more than double the memory that
the current printk implementation requires. This is because dictionaries
and meta-data are now stored separately.

If the old defaults (LOG_BUF_SHIFT=17 LOG_CPU_MAX_BUF_SHIFT=12) were
chosen because they are maximally acceptable defaults, then the defaults
should be reduced by 1 so that the final size is "similar" to the
current implementation.

If instead the defaults are left as-is, a machine with less than 64 CPUs
will reserve 336KiB for printk information (128KiB text, 128KiB
dictionary, 80KiB meta-data).

It might also be desirable to reduce the dictionary size (maybe 1/4 the
size of text?). However, since the new printk implementation allows for
non-intrusive dictionaries, we might see their usage increase and start
to be as large as the messages themselves.

John Ogness