Question about pdflush's two ratio?

[内核追踪 <kerneltrack@gmail.com>]

Hi, buddy, In these days, I'm watching about how the pdflush kernel
thread implemented. But we I saw there are two "*_ratio", I was
puzzled about these.

My kernel version is 2.6.9 and I found get_dirty_limits() in
page-writeback.c in KERNEL_SOURCE/mm.

After read the source and annotation about the function , I got that idea about
1. *dirty_ratio* is just for work with the number in system page
cache. It means if the (dirty pages number in system)/(total number
pages) is above the
    ratio , then we start pdflush.
2. *background_ratio* is just for every process. It means if there
were a single_process write pages and (its dirty pages) / (total
number pages in system)
    is above th ratio, then we also start pdflush.

This is my interpetation. But does it correct?

 Thx.
Jiancong