Re: [RFC PATCH] mm, compaction: allow compaction for GFP_NOFS requests

Dave Chinner
2016-10-13T00:29:52Z

On Fri, Oct 07, 2016 at 03:18:14PM +0200, Michal Hocko wrote:
> On Thu 06-10-16 13:11:42, Dave Chinner wrote:
> > On Wed, Oct 05, 2016 at 01:38:45PM +0200, Michal Hocko wrote:
> > > On Wed 05-10-16 07:32:02, Dave Chinner wrote:
> > > > On Tue, Oct 04, 2016 at 10:12:15AM +0200, Michal Hocko wrote:
> > > > > From: Michal Hocko <mhocko@suse.com>
> > > > >
> > > > > compaction has been disabled for GFP_NOFS and GFP_NOIO requests since
> > > > > the direct compaction was introduced by 56de7263fcf3 ("mm: compaction:
> > > > > direct compact when a high-order allocation fails"). The main reason
> > > > > is that the migration of page cache pages might recurse back to fs/io
> > > > > layer and we could potentially deadlock. This is overly conservative
> > > > > because all the anonymous memory is migrateable in the GFP_NOFS context
> > > > > just fine. This might be a large portion of the memory in many/most
> > > > > workkloads.
> > > > >
> > > > > Remove the GFP_NOFS restriction and make sure that we skip all fs pages
> > > > > (those with a mapping) while isolating pages to be migrated. We cannot
> > > > > consider clean fs pages because they might need a metadata update so
> > > > > only isolate pages without any mapping for nofs requests.
> > > > >
> > > > > The effect of this patch will be probably very limited in many/most
> > > > > workloads because higher order GFP_NOFS requests are quite rare,
> > > >
> > > > You say they are rare only because you don't know how to trigger
> > > > them easily. :/
> > >
> > > true
> > >
> > > > Try this:
> > > >
> > > > # mkfs.xfs -f -n size=64k <dev>
> > > > # mount <dev> /mnt/scratch
> > > > # time ./fs_mark -D 10000 -S0 -n 100000 -s 0 -L 32 \
> > > > -d /mnt/scratch/0 -d /mnt/scratch/1 \
> > > > -d /mnt/scratch/2 -d /mnt/scratch/3 \
> > > > -d /mnt/scratch/4 -d /mnt/scratch/5 \
> > > > -d /mnt/scratch/6 -d /mnt/scratch/7 \
> > > > -d /mnt/scratch/8 -d /mnt/scratch/9 \
> > > > -d /mnt/scratch/10 -d /mnt/scratch/11 \
> > > > -d /mnt/scratch/12 -d /mnt/scratch/13 \
> > > > -d /mnt/scratch/14 -d /mnt/scratch/15
> > >
> > > Does this simulate a standard or usual fs workload/configuration? I am
> >
> > Unfortunately, there was an era of cargo cult configuration tweaks
> > in the Ceph community that has resulted in a large number of
> > production machines with XFS filesystems configured this way. And a
> > lot of them store large numbers of small files and run under
> > significant sustained memory pressure.
>
> I see
>
> > I slowly working towards getting rid of these high order allocations
> > and replacing them with the equivalent number of single page
> > allocations, but I haven't got that (complex) change working yet.
>
> Definitely a good plan!
>
> Anyway I was playing with this in my virtual machine (4CPUs, 512MB of
> RAM split into two NUMA nodes). Started on a freshly created fs after
> boot, no other load in the guest. The performance numbers should be
> taken with grain of salt, though, because the host has 4CPUs as well and
> it wasn't completely idle, but should be OK enough to give us at least
> some picture. This is what fs_mark told me:
> Unpatched kernel:
> # Version 3.3, 16 thread(s) starting at Fri Oct 7 09:55:05 2016
> # Sync method: NO SYNC: Test does not issue sync() or fsync() calls.
> # Directories: Time based hash between directories across 10000 subdirectories with 180 seconds per subdirectory.
> # File names: 40 bytes long, (16 initial bytes of time stamp with 24 random bytes at end of name)
> # Files info: size 0 bytes, written with an IO size of 16384 bytes per write
> # App overhead is time in microseconds spent in the test not doing file writing related system calls.
> #
> FSUse% Count Size Files/sec App Overhead
> 1 1600000 0 4300.1 20745838
> 3 3200000 0 4239.9 23849857
> 5 4800000 0 4243.4 25939543
> 6 6400000 0 4248.4 19514050
> 8 8000000 0 4262.1 20796169
> 9 9600000 0 4257.6 21288675
> 11 11200000 0 4259.7 19375120
> 13 12800000 0 4220.7 22734141
> 14 14400000 0 4238.5 31936458
> 16 16000000 0 4231.5 23409901
> 18 17600000 0 4045.3 23577700
> 19 19200000 0 2783.4 58299526
> 21 20800000 0 2678.2 40616302
> 23 22400000 0 2693.5 83973996
> Ctrl+C because it just took too long.

Try running it on a larger filesystem, or configure the fs with more
AGs and a larger log (i.e. mkfs.xfs -f -dagcount=24 -l size=512m
<dev>). That will speed up modifications and increase concurrency.
This test should be able to run 5-10x faster than this (it
sustains 55,000 files/s @ 300MB/s write on my test fs on a cheap
SSD).

> while it doesn't seem to drop the Files/sec numbers starting with
> Count=19200000. I also see only a single
>
> [ 3063.815003] XFS: fs_mark(3272) possible memory allocation deadlock size 65624 in kmem_alloc (mode:0x2408240)

Remember that this is emitted only after /100/ consecutive
allocation failures. So the fact it is still being emitted means
that the situation is not drastically better....

> Unpatched kernel
> all orders
> begin:44.718798 end:5774.618736 allocs:15019288
> order > 0
> begin:44.718798 end:5773.587195 allocs:10438610
>
> Patched kernel
> all orders
> begin:64.612804 end:5794.193619 allocs:16110081 [107.2%]
> order > 0
> begin:64.612804 end:5794.193619 allocs:11741492 [112.5%]
>
> which would suggest that diving into the compaction rather than backing
> off and waiting for kcompactd to make the work for us was indeed a
> better strategy and helped the throughput.

Well, without a success/failure ratio being calculated it's hard to
tell what improvement it made. Did it increase the success rate, or
reduce failure latency so retries happened faster?

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com