Re: [RFC 0/3] Add madvise(..., MADV_WILLWRITE)

[Jan Kara <jack@suse.cz>]

On Fri 09-08-13 10:36:41, Andy Lutomirski wrote:
> On Fri, Aug 9, 2013 at 12:55 AM, Jan Kara <jack@suse.cz> wrote:
> > On Thu 08-08-13 15:58:39, Dave Hansen wrote:
> >> I was coincidentally tracking down what I thought was a scalability
> >> problem (turned out to be full disks :).  I noticed, though, that ext4
> >> is about 20% slower than ext2/3 at doing write page faults (x-axis is
> >> number of tasks):
> >>
> >> http://www.sr71.net/~dave/intel/page-fault-exts/cmp.html?1=ext3&2=ext4&hide=linear,threads,threads_idle,processes_idle&rollPeriod=5
> >>
> >> The test case is:
> >>
> >>       https://github.com/antonblanchard/will-it-scale/blob/master/tests/page_fault3.c
> >   The reason is that ext2/ext3 do almost nothing in their write fault
> > handler - they are about as fast as it can get. ext4 OTOH needs to reserve
> > blocks for delayed allocation, setup buffers under a page etc. This is
> > necessary if you want to make sure that if data are written via mmap, they
> > also have space available on disk to be written to (ext2 / ext3 do not care
> > and will just drop the data on the floor if you happen to hit ENOSPC during
> > writeback).
> 
> Out of curiosity, why does ext4 need to set up buffers?  That is, as
> long as the fs can guarantee that there is reserved space to write out
> the page, why isn't it sufficient to just mark the page dirty and let
> the writeback code set up the buffers?
  Well, because we track the fact that the space is reserved in the buffer
itself.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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