On 12/02/2015 02:13 AM, Brian Foster wrote:
On Tue, Dec 01, 2015 at 09:26:42PM +0200, Avi Kivity wrote:
On 12/01/2015 08:51 PM, Brian Foster wrote:
On Tue, Dec 01, 2015 at 07:09:29PM +0200, Avi Kivity wrote:
On 12/01/2015 06:29 PM, Brian Foster wrote:
On Tue, Dec 01, 2015 at 06:08:51PM +0200, Avi Kivity wrote:
On 12/01/2015 06:01 PM, Brian Foster wrote:
On Tue, Dec 01, 2015 at 05:22:38PM +0200, Avi Kivity wrote:
On 12/01/2015 04:56 PM, Brian Foster wrote:
On Tue, Dec 01, 2015 at 03:58:28PM +0200, Avi Kivity wrote:
On 12/01/2015 03:11 PM, Brian Foster wrote:
On Tue, Dec 01, 2015 at 11:08:47AM +0200, Avi Kivity wrote:
On 11/30/2015 06:14 PM, Brian Foster wrote:
On Mon, Nov 30, 2015 at 04:29:13PM +0200, Avi Kivity wrote:
On 11/30/2015 04:10 PM, Brian Foster wrote:
...
The case of waiting for I/O is much more worrying, because I/O latency are
much higher. But it seems like most of the DIO path does not trigger
locking around I/O (and we are careful to avoid the ones that do, like
writing beyond eof).
(sorry for repeating myself, I have the feeling we are talking past each
other and want to be on the same page)
Yeah, my point is just that just because the thread blocked on I/O,
doesn't mean the cpu can't carry on with some useful work for another
task.
In our case, there is no other task. We run one thread per logical core, so
if that thread gets blocked, the cpu idles.
The whole point of io_submit() is to issue an I/O and let the caller
continue processing immediately. It is the equivalent of O_NONBLOCK for
networking code. If O_NONBLOCK did block from time to time, practically all
modern network applications would see a huge performance drop.
Ok, but my understanding is that O_NONBLOCK would return an error code
in the blocking case such that userspace can do something else or retry
from a blockable context.
I did not mean the exact equivalent, but in the spirit of allowing a
thread to perform an I/O task (networking or file I/O) in parallel with
computation.
For networking, returning an error is fine because there exists a
notification (epoll) to tell userspace when a retry would succeed. For
file I/O, there isn't one. Still, returning an error is better than
nothing because then, as you say, you can retry in a blockable context.
I think this is similar to what hch posted wrt
to the pwrite2() bits for nonblocking buffered I/O or what I was asking
about earlier on with regard to returning an error if some blocking
would otherwise occur.
Yes. Anything except silently blocking!
We submit an I/O which is
asynchronous in nature and wait on a completion, which causes the cpu to
schedule and execute another task until the completion is set by I/O
completion (via an async callback). At that point, the issuing thread
continues where it left off. I suspect I'm missing something... can you
elaborate on what you'd do differently here (and how it helps)?
Just apply the same technique everywhere: convert locks to trylock +
schedule a continuation on failure.
I'm certainly not an expert on the kernel scheduling, locking and
serialization mechanisms, but my understanding is that most things
outside of spin locks are reschedule points. For example, the
wait_for_completion() calls XFS uses to wait on I/O boil down to
schedule_timeout() calls. Buffer locks are implemented as semaphores and
down() can end up in the same place.
But, for the most part, XFS seems to be able to avoid sleeping. The call to
__blockdev_direct_IO only launches the I/O, so any locking is only around
cpu operations and, unless there is contention, won't cause us to sleep in
io_submit().
Trying to follow the code, it looks like xfs_get_blocks_direct (and
__blockdev_direct_IO's get_block parameter in general) is synchronous, so
we're just lucky to have everything in cache. If it isn't, we block right
there. I really hope I'm misreading this and some other magic is happening
elsewhere instead of this.
Nope, it's synchronous from a code perspective. The
xfs_bmapi_read()->xfs_iread_extents() path could have to read in the
inode bmap metadata if it hasn't been done already. Note that this
should only happen once as everything is stored in-core, so in most
cases this is skipped. It's also possible extents are read in via some
other path/operation on the inode before an async I/O happens to be
submitted (e.g., see some of the other xfs_bmapi_read() callers).
Is there (could we add) some ioctl to prime this cache? We could call it
from a worker thread where we don't mind blocking during open.
I suppose that's possible, or the worker thread could perform some
existing operation known to prime the cache. I don't think it's worth
getting into without a concrete example, however. The extent read
example we're batting around might not ever be a problem (as you've
noted due to file size), if files are truncated and recycled, for
example.
What is the eviction policy for this cache? Is it simply the block
device's page cache?
IIUC the extent list stays around until the inode is reclaimed. There's
a separate buffer cache for metadata buffers. Both types of objects
would be reclaimed based on memory pressure.
It comes down to size of disk, size of memory, and average file size. I
expect that with current disk and memory sizes the metadata is quite
small, so this might not be a problem, and even a cold start would
self-prime in a reasonably short time.
What about the write path, will we see the same problems there? I would
guess the problem is less severe there if the metadata is written with
writeback policy.
Metadata is modified in-core and handed off to the logging
infrastructure via a transaction. The log is flushed to disk some time
later and metadata writeback occurs asynchronously via the xfsaild
thread.
Unless, I expect, if the log is full. Since we're hammering on the disk
quite heavily, the log would be fighting with user I/O and possibly losing.
Does XFS throttle user I/O in order to get the log buffers recycled faster?
Is there any way for us to keep track of it, and reduce disk pressure
when it gets full?
Oh you answered that already, /sys/fs/xfs/device/log/*.
Brian
Either way, the extents have to be read in at some point and I'd expect
that cpu to schedule onto some other task while that thread waits on I/O
to complete (read-ahead could also be a factor here, but I haven't
really dug into how that is triggered for buffers).
To provide an example, our application, which is a database, faces this
problem exact at a higher level. Data is stored in data files, and data
items' locations are stored in index files. When we read a bit of data, we
issue an index read, and pass it a continuation to be executed when the read
completes. This latter continuation parses the data and passes it to the
code that prepares it for merging with data from other data files, and an
eventual return to the user.
Having written code for over a year in this style, I've come to expect it to
be used everywhere asynchronous I/O is used, but I realize it is fairly hard
without good support from a framework that allows continuations to be
composed in a natural way.
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