makedumpfile 1.5.0 takes much more time to dump

[HATAYAMA Daisuke]

From: Vivek Goyal <vgoyal at redhat.com>
Subject: makedumpfile 1.5.0 takes much more time to dump
Date: Thu, 20 Sep 2012 16:06:34 -0400

> Hi Atsushi san,
> 
> We tried makedumpfile 1.5.0 on a 1TB machine and it seems to regress
> badly. We reserved 192MB of memory and following are test results.
> 
> #1. makedumpfile-1.4.2 -E --message-level 1 -d 31
> real     3m47.520s
> user     0m56.543s
> sys	 2m41.631s
> 
> #2. makedumpfile-1.5.0 -E --message-level 1 -d 31
> real     52m25.262s
> user     32m51.310s
> sys	 18m53.265s
> 
> #3. makedumpfile-1.4.2 -c --message-level 1 -d 31
> real     8m49.107s
> user     4m34.180s
> sys	 4m8.691s
> 
> #4. makedumpfile-1.5.0 -c --message-level 1 -d 31
> real     46m48.985s
> user     29m35.203s
> sys	 16m43.149s
> 

Hello Vivek,

On v1.5.0 we cannot filter free pages in constant space becuase we
have yet to test it. Instead, the existing method is used here, which
repeats walking on a whole page frames the number of cycles times.

As Kumagai-san explains, the number of cycles can be calculated by the
following expression:

  N = physical memory size / (page size * bit per byte(8) * BUFSIZE_CYCLIC)

So,

  N = 2TB / (4KB * 8 * 1MB) = 64 cycles.

I guess on this environment, it took about 50 seconds to filter free
pages in one cycle.

Thanks.
HATAYAMA, Daisuke