On 12/13/12 18:25, scameron@xxxxxxxxxxxxxxxxxx wrote:
On Thu, Dec 13, 2012 at 04:22:33PM +0100, Bart Van Assche wrote:
On 12/11/12 01:00, scameron@xxxxxxxxxxxxxxxxxx wrote:
The driver, like nvme, has a submit and reply queue per cpu.
This is interesting. If my interpretation of the POSIX spec is correct
then aio_write() allows to queue overlapping writes and all writes
submitted by the same thread have to be performed in the order they were
submitted by that thread. What if a thread submits a first write via
aio_write(), gets rescheduled on another CPU and submits a second
overlapping write also via aio_write() ? If a block driver uses one
queue per CPU, does that mean that such writes that were issued in order
can be executed in a different order by the driver and/or hardware than
the order in which the writes were submitted ?
See also the aio_write() man page, The Open Group Base Specifications
Issue 7, IEEE Std 1003.1-2008
(http://pubs.opengroup.org/onlinepubs/9699919799/functions/aio_write.html).
It is my understanding that the low level driver is free to re-order the
i/o's any way it wants, as is the hardware. It is up to the layers above
to enforce any ordering requirements. For a long time there was a bug
in the cciss driver that all i/o's submitted to the driver got reversed
in order -- adding to head of a list instead of to the tail, or vice versa,
I forget which -- and it caused no real problems (apart from some slight
performance issues that were mostly masked by the Smart Array's cache.
It was caught by firmware guys noticing LBAs coming in in weird orders
for supposedly sequential workloads.
So in your scenario, I think the overlapping writes should not be submitted
by the block layer to the low level driver concurrently, as the block layer
is aware that the lld is free to re-order things. (I am very certain
that this is the case for scsi low level drivers and block drivers using a
request_fn interface -- less certain about block drivers using the
make_request interface to submit i/o's, as this interface is pretty new
to me.
As far as I know there are basically two choices:
1. Allow the LLD to reorder any pair of write requests. The only way
for higher layers to ensure the order of (overlapping) writes is then
to separate these in time. Or in other words, limit write request
queue depth to one.
2. Do not allow the LLD to reorder overlapping write requests. This
allows higher software layers to queue write requests (queue depth
> 1).
From my experience with block and SCSI drivers option (1) doesn't look
attractive from a performance point of view. From what I have seen
performance with QD=1 is several times lower than performance with QD >
1. But maybe I overlooked something ?
Bart.
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