linux-kernel - Re: [PATCH 0/2] printk: replace ringbuffer

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Message-ID: <87sgjp9foj.fsf@linutronix.de>
Date:   Wed, 05 Feb 2020 17:12:12 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     lijiang <lijiang@...hat.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] printk: replace ringbuffer

On 2020-02-05, lijiang <lijiang@...hat.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