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

Dave Hansen
2013-08-12T22:44:15Z

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'm not saying ext4 write fault path cannot possibly be optimized (noone
> seriously looked into that AFAIK so there may well be some low hanging
> fruit) but it will always be slower than ext2/3. A more meaningful
> comparison would be with filesystems like XFS which make similar guarantees
> regarding data safety.

ext4 beats xfs from what I can tell. I ran with fewer steps to make the
testing faster, which is to blame for the stair-stepping, btw...