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

Andy Lutomirski
2013-08-09T17:44:40Z

On Fri, Aug 9, 2013 at 10:42 AM, Dave Hansen <dave.hansen@intel.com> wrote:
> On 08/09/2013 12:55 AM, Jan Kara 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).
>
> I did try throwing a fallocate() in there to see if it helped. It
> didn't appear to help. Should it have?

Try reading all the pages after mmap (and keep the fallocate).

In theory, MAP_POPULATE should help some, but until Linux 3.9
MAP_POPULATE was a disaster, and I'm still a bit afraid of it.

--Andy

--
Andy Lutomirski
AMA Capital Management, LLC