[vdr] Re: Recordings menu sluggish - *the* solution.

["Dr. Werner Fink" <werner@suse.de>]

On Fri, Oct 03, 2003 at 03:48:24PM +0200, Carsten Koch wrote:
> Hi,
> 
> we have discussed this subject here before.
> I believe we consent that the main reason for the big delays
> when opening the recordings menu is the fact that vdr reads
> and writes huge amounts of data from/to the nnn.vdr files,
> the kernel tries to cache these, thus moving much more useful
> stuff (like cached directory entries and cached small files)
> out of the memory cache.
> It usually makes no sense at all to cache the nnn.vdr files,
> as they normally are read/written only once.
> 
> I recently found out that kernels > 2.4.9 seem to have exactly
> the solution we were looking for.
> 
> From "man 2 open":
> 
>        O_DIRECT
>               Try to minimize cache effects of  the  I/O  to  and
>               from  this file.  In general this will degrade per­
>               formance, but it is useful in  special  situations,
>               such  as  when  applications  do their own caching.
>               File  I/O  is  done  directly  to/from  user  space
>               buffers.  The I/O is synchronous, i.e., at the com­
>               pletion of the read(2)  or  write(2)  system  call,
>               data   is  guaranteed  to  have  been  transferred.
>               Transfer sizes, and the alignment  of  user  buffer
>               and  file offset must all be multiples of the logi­
>               cal block size of the file system.
>               This flag is supported on  a  number  of  Unix-like
>               systems;  support  was  added under Linux in kernel
>               version 2.4.10.
>               A semantically similar interface for block  devices
>               is described in raw(8).
> 
> 
> I guess a patch that uses this should be trivial.

If you're using O_DIRECT you've to provide the thread which writes
out the stuff in background.  In other words: you need a ringbuffer
an own thread and the interface for your application.

If you not do this the application has to wait (e.g. a big DB using
O_DIRECT for secure data transfer) upto the point where teh data has 
be put onto the physical sector of your disk ;)

> I have not tried it yet.
> Is there anything obvious that speaks against this being a nice
> solution fo our problem?


    Werner


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