Re: sleeps and waits during io_submit

[Dave Chinner <david@fromorbit.com>]

On Wed, Dec 02, 2015 at 11:02:08AM +0200, Avi Kivity wrote:
> On 12/02/2015 01:06 AM, Dave Chinner wrote:
> >On Tue, Dec 01, 2015 at 11:38:29PM +0200, Avi Kivity wrote:
> >>On 12/01/2015 11:19 PM, Dave Chinner wrote:
> >>>>>XFS spread files across the allocation groups, based on the directory these
> >>>>>files are created,
> >>>>Idea: create the files in some subdirectory, and immediately move
> >>>>them to their required location.
....
> >>My hack involves creating the file in a random directory, and while
> >>it is still zero sized, move it to its final directory.  This is
> >>simply to defeat the ag selection heuristic.
> >Which you really don't want to do.
> 
> Why not?  For my directory structure, files in the same directory do
> not share temporal locality.  What does the ag selection heuristic
> give me?

Wrong question. The right question is this: what problems does
subverting the AG selection heuristic cause me?

If you can't answer that question, then you can't quantify the risks
involved with making such a behavioural change.

> >>>>>  trying to keep files as close as possible from their
> >>>>>metadata.
> >>>>This is pointless for an SSD. Perhaps XFS should randomize the ag on
> >>>>nonrotational media instead.
> >>>Actually, no, it is not pointless. SSDs do not require optimisation
> >>>for minimal seek time, but data locality is still just as important
> >>>as spinning disks, if not moreso. Why? Because the garbage
> >>>collection routines in the SSDs are all about locality and we can't
> >>>drive garbage collection effectively via discard operations if the
> >>>filesystem is not keeping temporally related files close together in
> >>>it's block address space.
> >>In my case, files in the same directory are not temporally related.
> >>But I understand where the heuristic comes from.
> >>
> >>Maybe an ioctl to set a directory attribute "the files in this
> >>directory are not temporally related"?
> >And exactly what does that gain us?
> 
> I have a directory with commitlog files that are constantly and
> rapidly being created, appended to, and removed, from all logical
> cores in the system.  Does this not put pressure on that allocation
> group's locks?

Not usually, because if an AG is contended, the allocation algorithm
skips the contended AG and selects the next uncontended AG to
allocate in. And given that the append algorithm used by the
allocator attempts to use the last block of the last extent as the
target for the new extent (i.e. contiguous allocation) once a file
has skipped to a different AG all allocations will continue in that
new AG until it is either full or it becomes contended....

IOWs, when AG contention occurs, the filesystem automatically
spreads out the load over multiple AGs. Put simply, we optimise for
locality first, but we're willing to compromise on locality to
minimise contention when it occurs. But, also, keep in mind that
in minimising contention we are still selecting the most local of
possible alternatives, and that's something you can't do in
userspace....

> >Exactly what problem are you
> >trying to solve by manipulating file locality that can't be solved
> >by existing knobs and config options?
> 
> I admit I don't know much about the existing knobs and config
> options.  Pointers are appreciated.

You can find some work in progress here:

https://git.kernel.org/cgit/fs/xfs/xfs-documentation.git/

looks like there's some problem with xfs.org wiki, so the links
to the user/training info on this page:

http://xfs.org/index.php/XFS_Papers_and_Documentation

aren't working.

> >Perhaps you'd like to read up on how the inode32 allocator behaves?
> 
> Indeed I would, pointers are appreciated.

Inode allocation section here:

https://git.kernel.org/cgit/fs/xfs/xfs-documentation.git/tree/admin/XFS_Performance_Tuning/filesystem_tunables.asciidoc

> >Once we know which of the different algorithms is causing the
> >blocking issues, we'll know a lot more about why we're having
> >problems and a better idea of what problems we actually need to
> >solve.
> 
> I'm happy to hack off the lowest hanging fruit and then go after the
> next one.  I understand you're annoyed at having to defend against
> what may be non-problems; but for me it is an opportunity to learn
> about the file system.

No, I'm not annoyed. I just don't want to be chasing ghosts and so
we need to be on the same page about how to track down these issues.
And, beleive me, you'll learn a lot about how the filesystem behaves
just by watching how the different configs react to the same
input...

> For us it is the weakest spot in our system,
> because on the one hand we heavily depend on async behavior and on
> the other hand Linux is notoriously bad at it.  So we are very
> nervous when blocking happens.

I can't disagree with you there - we really need to fix what we can
within the constraints of the OS first, then we once we have it
working as well as we can, then we can look to solving the remaining
"notoriously bad" AIO problems...

> >>>effectively than lots of little trims (i.e. one per file) that the
> >>>drive cannot do anything useful with because they are all smaller
> >>>than the internal SSD page/block sizes and so get ignored.  This is
> >>>one of the reasons fstrim is so much more efficient and effective
> >>>than using the discard mount option.
> >>In my use case, the files are fairly large, and there is constant
> >>rewriting (not in-place: files are read, merged, and written back).
> >>So I'm worried an fstrim can happen too late.
> >Have you measured the SSD performance degradation over time due to
> >large overwrites? If not, then again it is a good chance you are
> >trying to solve a theoretical problem rather than a real problem....
> >
> 
> I'm not worried about that (maybe I should be) but about the SSD
> reaching internal ENOSPC due to the fstrim happening too late.
> 
> Consider this scenario, which is quite typical for us:
> 
> 1. Fill 1/3rd of the disk with a few large files.
> 2. Copy/merge the data into a new file, occupying another 1/3rd of the disk.
> 3. Repeat 1+2.
>
> If this is repeated few times, the disk can see 100% of its space
> occupied (depending on how free space is allocated), even if from a
> user's perspective it is never more than 2/3rds full.

I don't think that's true. SSD behaviour largely depends on how much
of the LBA space has been written to (i.e. marked used) and so that
metric tends to determine how the SSD behaves under such workloads.
This is one of the reasons that overprovisioning SSD space (e.g.
leaving 25% of the LBA space completely unused) results in better
performance under overwrite workloads - there's lots more scratch
space for the garbage collector to work with...

Hence as long as the filesystem is reusing the same LBA regions for
the files, TRIM will probably not make a significant difference to
performance because there's still 1/3rd of the LBA region that is
"unused". Hence the overwrites go into the unused 1/3rd of the SSD,
and the underlying SSD blocks associated with the "overwritten" LBA
region are immediately marked free, just like if you issued a trim
for that region before you start the overwrite.

With the way the XFS allocator works, it fills AGs from lowest to
highest blocks, and if you free lots of space down low in the AG
then that tends to get reused before the higher offset free space.
hence the XFS allocates space in the above workload would result in
roughly 1/3rd of the LBA space associated with the filesystem
remaining unused. This is another allocator behaviour designed for
spinning disks (to keep the data on the faster outer edges of
drives) that maps very well to internal SSD allocation/reclaim
algorithms....

FWIW, did you know that TRIM generally doesn't return the disk to
the performance of a pristine, empty disk?  Generally only a secure
erase will guarantee that a SSD returns to "empty disk" performance,
but that also removes all data from then entire SSD.  Hence the
baseline "sustained performance" you should be using is not "empty
disk" performance, but the performance once the disk has been
overwritten completely at least once. Only them will you tend to see
what effect TRIM will actually have.

> Maybe a simple countermeasure is to issue an fstrim every time we
> write 10%-20% of the disk's capacity.

Run the workload to steady state performance and measure the
degradation as it continues to run and overwrite the SSDs
repeatedly. To do this properly you are going to have to sacrifice
some SSDs, because you're going to need to overwrite them quite a
few times to get an idea of the degradation characteristics and
whether a periodic trim makes any difference or not.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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