linux-kernel - Re: do_generic_mapping

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Date:	Mon, 12 Mar 2007 16:13:55 +0100
From:	Jan Kara <jack@...e.cz>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Ashif Harji <asharji@...uwaterloo.ca>, linux-kernel@...r.kernel.org
Subject: Re: do_generic_mapping_read performance issue

On Mon 12-03-07 15:39:00, Nick Piggin wrote:
> On Mon, Mar 12, 2007 at 03:20:12PM +0100, Jan Kara wrote:
> >   Hi,
> > 
> > > Hi, I am encountering a performance problem, which I have tracked into the 
> > > Linux kernel. The problem occurs with my experimental web server that uses 
> > > sendfile to repeatedly transmit files.  The files are based on the static 
> > > portion of the SPECweb99 fileset and range in size to model a reasonable 
> > > workload.  With this workload, a significant number of the requests are 
> > > for files of size 4 KB or less.
> > > 
> > > I have determined that the performance problems occurs in the function
> > > do_generic_mapping_read in file mm/filemap.c for kernel version 2.6.20.1.
> > > Here is the specific code fragment:
> > > 
> > >         /*
> > >          * When (part of) the same page is read multiple times
> > >          * in succession, only mark it as accessed the first time.
> > >          */
> > >         if (prev_index != index)
> > >                 mark_page_accessed(page);
> >   Actually, the code is like that certainly for two years :).
> 
> Did it always use ra->prev_page? ISTR it using pos%PAGE_SIZE == 0 at some
> stage (ie. read from the start of a page -- obviously that also has holes).
  Yes, at least in 2.6.12-rc5 which is the first one in git :).

> > > I was wondering if anyone could explain why the call to mark_page_accessed 
> > > is conditional? That is, what problem it is trying to solve. It would seem 
> > > that in many scenarios, if the same page is accessed repeatedly, then it 
> > > would be appropriate to keep that page cached.
> >   I also don't know why the condition is there but it's there at least
> > for two years so I'm not sure anybody remembers ;). Nick, do you have
> > an idea?
> 
> Yeah it is there because that is basically how our "use once" detection
> handles the case where an app does not read in chunks that are PAGE_SIZE
> multiples and PAGE_SIZE aligned.
  OK, I see. Then I'm not sure the check does more good than bad. Because
if we happen to reread the same chunk several times, then the check does a
wrong thing...

									Honza
-- 
Jan Kara <jack@...e.cz>
SuSE CR Labs
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