Re: Application, dll and driver design

From: Raj (Raj_at_discussions.microsoft.com)
Date: 08/17/04 

Date: Tue, 17 Aug 2004 06:27:07 -0700

How big can semaphore "lMaximumCount" can be. Is there any internal overhead
associated with bigger "lMaximumCount". Obviously my queue must be as big as
lMaximumCount. I am planning to have it as big as 250. Is it typical to have
bigger lMaximumCount.

"Raj" wrote:

>
> Thank you very much for your detailed reply. I agree with you.
>
> But there is one problem with replacing window messages with packet reading
> thread. I have 15 processes all talking to each other and Dll process. If I
> replace with packet reading thread, each of the process should have 'PRT
> semaphore' handle of each other. It would mean 15*15 handles. When one of the
> process is exiting, all other processes should clear their handle "in their
> context". So cleaning up will be exhaustive race prone cases. If one of
> process is terminated, even bigger headache. I can implement this scheme, but
> should be very careful with design. Finally I came to some idea.
>
> Each process will have handle of other process PRT handle only if they are
> talking. All may not talk to every other process. Even if there are 15*15
> handles, I am willing to do so if there is tangible performance benifit.
> Every handle also has a status variable attached to it. When a process is
> exiting, it will set this status state to invalid in the shared memory of
> these handles. It will then post a message to Dll and clears off. Dll will
> post a message to all other apps to clear handle in their context. So much
> work to replace standard interface.
>
> Looks like there is performance difference. I will test it and let you know
> the results. Any comments appreciated.
>
>
> "Le Chaud Lapin" wrote:
>
> > Raj <Raj@discussions.microsoft.com> wrote in message news:<2F1BC142-0ED1-470F-884D-E5ACD4ECFC47@microsoft.com>...
> > > Need some advice on overall design issue.
> > >
> > > My Dll process can support upto 15 applications. These applications will
> > > receive data from remote PC on some transport like USB. My driver collects
> > > this data and using pending IRPs and IOCTL calls, dll will pull the data from
> > > driver and post 'DATA_RECEIVED" windows message(with packet IDentifier) to
> > > respective Application. Application will call dll API with packet ID and
> > > receive data. Shared memory is used IOCTL buffers and dll API copies data to
> > > App buffer from this shared buffer pool. I use 20 buffers each 10K size. One
> > > 10K buffer is sent in each IOCTL call to pull as many packets as fit. All
> > > 200K belongs to shared memory at user mode memory mapped file.
> > >
> > > Every thing works fine. But I observe lack of speed when small packets are
> > > used and mega bytes of data sent. Speed difference is low as a factor of 20
> > > between 32K packet and 2K packets.
> >
> > If I understand you correctly, a device-interface-thread (DIT) in the
> > DLL is blocking on a call into the driver to get the data via IOCTL
> > whereby it copies received packets into one of the 10K buffers, then
> > posts a message to the appropriate window, after which window
> > procedure calls back into the DLL to copy the data *again* to the app
> > buffer.
> >
> > If so, try this:
> >
> > First, get rid of the window message passing. There is a lot of
> > machinery involved in passing message to windows, including multiples
> > calls against mutexes. Instead, put a packet-reader-thread (PRT) in
> > each app that blocks while trying to read from the shared buffer of
> > the DLL. Blocking will persist until the DIT of the DLL raises a
> > semaphore for the appropriate PRT. The PRT will then unblock and scan
> > for packets in the packet pool, process them, and clear a flag for the
> > 10K slot to say that that slot is free. However, it should be evident
> > that if you have this flag on a per-10K buffer basis, this method will
> > not work, as two different PRT's might have incoming packets residing
> > in the same 10K buffer. The solution is to make your buffers smaller,
> > much less than 10K of course, but up to the maxium size of a packet,
> > then when your DLL thread goes to read into the pool in the IOCTL,
> > specify the indexes accross the user/kernel transistion (array of
> > unsigned int might work) of the slots that are available for filling.
> > Of course, if the shared section is available at ring-0, a hard scan
> > in a tight for loop will work. The PRT blocking semaphores will need
> > names so that you can reference them accross process boundaries (if
> > you so choose). Also note that if your apps will be running as
> > services before log in, you should supply a NULL security descriptor
> > in call to CreateSemaphore. Finally, if packets cannot be processed
> > immediately after they arrive, but must be queued, you should still
> > try to avoid copying if posible, perhaps by maintaining a (protected
> > by mutex) queue of pointers to packet in each app, the pointers
> > pointing to packets that have been "acknowledge for later processing".
> > If you detect that this is not feasible (look up Little's Theoreom on
> > Internet to determine whether it is or not based on expected receive
> > rate and expected time for processing), then you should increase the
> > size of your packet pool, adding a bit for expected deviation, to
> > avoid overflow. If this is not possible, then go ahead and do the copy
> > to a separate app buffer, but use memcpy or inline assembly code to
> > effect the copy - they are much faster than a run-of-the-mill for
> > loop.
> >
> > Doing these things will not eliminate the context switching over head,
> > but it will reduce the amount of the system spends working with thread
> > queues.
> >
> > -Chaud Lapin-
> >

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