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

Dave Hansen
2013-08-09T17:42:21Z

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?