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:
On Tue, Dec 01, 2015 at 09:07:14PM +0200, Avi Kivity wrote:
On 12/01/2015 08:03 PM, Carlos Maiolino wrote:
Hi Avi,
else is going to execute in our place until this thread can make
progress.
For us, nothing else can execute in our place, we usually have exactly one
thread per logical core. So we are heavily dependent on io_submit not
sleeping.
The case of a contended lock is, to me, less worrying. It can be reduced by
using more allocation groups, which is apparently the shared resource under
contention.
I apologize if I misread your previous comments, but, IIRC you said you can't
change the directory structure your application is using, and IIRC your
application does not spread files across several directories.
I miswrote somewhat: the application writes data files and commitlog
files. The data file directory structure is fixed due to
compatibility concerns (it is not a single directory, but some
workloads will see most access on files in a single directory. The
commitlog directory structure is more relaxed, and we can split it
to a directory per shard (=cpu) or something else.
If worst comes to worst, we'll hack around this and distribute the
data files into more directories, and provide some hack for
compatibility.
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.
See xfs_fsr.
Can you elaborate? I don't see how it is applicable.
Just pointing out that this is what xfs_fsr does to control locality
of allocation for files it is defragmenting. Except that rather than
moving files, it uses XFS_IOC_SWAPEXT to switch the data between two
inodes atomically...
Ok, thanks.
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?
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?
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.
Perhaps you'd like to read up on how the inode32 allocator behaves?
Indeed I would, pointers are appreciated.
e.g. If the files in a directory are all close together, and the
directory is removed, we then leave a big empty contiguous region in
the filesystem free space map, and when we send discards over that
we end up with a single big trim and the drive handles that far more
Would this not be defeated if a directory that happens to share the
allocation group gets populated simultaneously?
Sure. But this sort of thing is rare in the real world, and when
they do occur, it generally only takes small tweaks to algorithms
and layouts make them go away. I don't care to bikeshed about
theoretical problems - I'm in the business of finding the root cause
of the problems users are having and solving those problems. So far
what you've given us is a ball of "there's blocking in AIO
submission", and the only one that is clear cut is the timestamp
update.
Go back and categorise the types of blocking that you are seeing -
whether it be on the AGIs during inode manipulation, one the AGFs
becuse of concurrent extent allocation, on log forces because of
slow discards in the transcation completion, on the transaction
subsystem because of a lack of log space for concurrent
reservations, etc. And then determine if changing the layout of the
filesystem (e.g. number of AGs, size of log, etc) and different
mount options (e.g. turning off discard, using inode32 allocator,
etc) make any difference to the blocking issues you are seeing.
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. 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.
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.
Maybe a simple countermeasure is to issue an fstrim every time we write
10%-20% of the disk's capacity.
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