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Re: [Qemu-devel] [PATCH v4 0/9] Make 'dump-guest-memory' dump in kdump-c

From: Qiao Nuohan
Subject: Re: [Qemu-devel] [PATCH v4 0/9] Make 'dump-guest-memory' dump in kdump-compressed format
Date: Fri, 28 Jun 2013 10:57:28 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.5) Gecko/20120607 Thunderbird/10.0.5

On 06/27/2013 04:54 PM, Stefan Hajnoczi wrote:

The interesting question is how effective this approach is.  If it's
good enough then it would be a fairly simple modification to dump.c.

I see, if excluding zero page in ELF can make a lot of size reduce, it's better
to choose this method. But think over the situation that kernel is on for a
long time, then few zero pages will be in memory, compression will do more work
to reduce size not excluding zero pages. So the approach is not always

A test on a 1GB memory, and the machine is just on:

size       format         method for reducing memory

1.1GB      ELF            no
1.1GB      kdump          no
227MB      kdump          with all zero pages excluded
 96MB      kdump          compressed with zero pages remained
 88MB      kdump          compressed with zero pages excluded

excluding zero pages does some work, but compression seems to be more effective.

I meant simply compressing the ELF output during creation.  But I guess
the tools need random access to the ELF file, which is inefficient with
zlib and friends.

It is the reason of why I don't like entirely compression. It will make crash work in terrific bad performance.

I'm not happy with duplicating the kdump FILEDUMP format code into QEMU
because it is relatively big and ugly (temporary files make up a large
part of that).

If it makes the code simpler and smaller it would be nice.

That's the point. I will make the code simpler.

Thanks for your comments!

Qiao Nuohan

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