- Pending IOCTLs and Slow User Mode Response
- Posted by Soquel Dude on July 2nd, 2005
I've written an NDIS IM driver that forwards notifications to a user-mode
app via a pended IOCTL. The app has 32 threads that simply call
DeviceIoControl and then queue the notification up for another consumer
thread.
The problem that I'm seeing is that under heavy stress, where the driver
needs to notify the app, I see the number of IOCTLs plumbed by the app stop,
with the result that the driver runs out of pending IOCTLS/IRPs.
Each thread of the user mode app calls DeviceIoControl() in a non-overlapped
manner. Would calling this with an overlapped structure (and opening the
file with FILE_FLAG_OVERLAPPED) help?
- Posted by Maxim S. Shatskih on July 2nd, 2005
Threads doing IO calls on the same file handle are serialized if the IO is
not overlapped. So, 32 threads or 1 - no effect 
Use overlapped IO.
--
Maxim Shatskih, Windows DDK MVP
StorageCraft Corporation
maxim@storagecraft.com
http://www.storagecraft.com
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- Posted by poltrone on July 2nd, 2005
Hi Maxim,
i never heard of this before and i'm quite curious about that.
Are you saying that the IO manager won't let a synchronous DeviceIOControl
pass through to the driver if another thread using the same handle has
a synchronous DeviceIOControl pending?
poltrone
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- Posted by Don Burn on July 2nd, 2005
If you think about this you will see that OS has to do if the way Max
describes. If you do not use OVERLAPPED I/O you are expecting things to
occur in the order you request them. Since drivers are inherently
asynchronous this order has to be enforced by the upper layers of the OS, by
releasing only one I/O at a time to a driver.
Note, never try to rely on this in your driver, you have no control over a
users ability to use OVERLAPPED I/O so designing for synchronous I/O is
always a bug.
--
Don Burn (MVP, Windows DDK)
Windows 2k/XP/2k3 Filesystem and Driver Consulting
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- Posted by Maxim S. Shatskih on July 2nd, 2005
Yes. Exactly. Same is true on read/write.
--
Maxim Shatskih, Windows DDK MVP
StorageCraft Corporation
maxim@storagecraft.com
http://www.storagecraft.com
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