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

[Klaus Schmidinger <Klaus.Schmidinger@cadsoft.de>]

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

The only thing that might be a problem is

        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 might make things more complicated than just setting
the O_DIRECT flag.

Apart from that the idea seems good, since then the system
probably wouldn't lose the directory information.

Klaus


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