Re: [PATCH 01/16] mm: delayed page activation

[Wu Fengguang <wfg@mail.ustc.edu.cn>]

On Wed, Dec 07, 2005 at 03:44:25PM +0300, Nikita Danilov wrote:
> Wu Fengguang writes:
>  > Yes, it also increased the lifetimes by meaningful values: first re-accessed
>  > pages are prolonged more lifetime. Immediately removing them from inactive_list 
>  > is basicly doing MRU eviction.
> 
> Are you talking about CLOCK-pro here? I don't understand your statement
> in the context of current VM: if the "first re-accessed" page was close
> to the cold tail of the inactive list, and "second re-accessed" page was
> close to the head of the inactive list, then life-time of second one is
> increased by larger amount.

Sorry, I fail to mention that I'm comparing two pages that are read in at the
same time, therefore they are in the same place in inactive_list. But their
re-access time can be quite different.

There are roughly two kinds of reads: almost instantly and slowly forward. For
the former one, read-in-time = first-access-time, unless for initial cache misses.
The latter one is the original purpose of of the patch: to keep one chunk of
read-ahead pages together, instead of let them littering throughout the lru
list.

>  > Delayed activation increased scanning activity, while immediate activation
>  > increased the locking activity. Early profiling data on a 2 CPU Xeon box showed
>  > that the delayed activation acctually cost less time.
> 
> That's great, but current mark_page_accessed() has an important
> advantage: the work is done by the process that accessed the page in
> read/write path, or at page fault. By delegating activation to the VM
> scanner, the burden of work is shifted to the innocent thread that
> happened to trigger scanning during page allocation.

Thanks to notice it. It will happen in the direct page reclaim path. But I have
just made interesting tests of the patch, in which direct page reclaims were
reduced to zero. Till now I have no hint of why this is happening :)

Wu