Query about cyclic buffer size

Atsushi Kumagai kumagai-atsushi at mxc.nes.nec.co.jp
Thu Sep 20 03:25:35 EDT 2012

Hello Vivek,

On Thu, 13 Sep 2012 10:10:18 -0400
Vivek Goyal <vgoyal at redhat.com> wrote:

> Hi Atsushi-san,
> How is size of cyclic buffer decided with 1.5.0? In practice I don't think
> asking user for the size is a good idea as user does not know how much
> memory will be left after kernel boot.

As you said, I don't think asking user for the size is a smart way, either.
So, I agree with your basic idea explained in below.

> Will it make sense for makedumpfile to automatically determine the buffer
> size based on amount of free memory available in the system. Something
> like use 70-80% of free memory as cyclic buffer.

Now, I think makedumpfile should automatically choose the lesser of the two below
as the size of cyclic buffer when the size isn't specified:

  a. the size enough for storing the 1st/2nd bitmap for the whole of vmcore
  b. the limit size (e.g. 70-80% of free memory)

However, I don't have a good idea how to determine the appropriate size
for the case of "b". 
If someone have a idea for the limit size, please let me know.

By the way, I'll be on long vacation until Sep 30, so I can't reply.

Atsushi kumagai

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