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

[Andreas Trauer <vdr@trauers.homelinux.net>]

Carsten Koch wrote:

> Yes, but that solution is complicated as well and it would be a
> wasted effort after implementing O_DIRECT. And O_DIRECT gives
> us 300% of the usefulness:
>
> *  O_DIRECT solves the "Recordings menu sluggish" problem in an
>    elegant way.
>
> *  It makes all of VDR I/O on nnn.vdr files faster, as the kernel
>    can DMA directly between the device and the VDR buffer.
>
> *  It increases the performance of the entire system, by eliminating
>    the overhead of moving gigabytes of data in and out of the cache
>    needlessly and by keeping the cache memory available for useful data.
>
> Carsten.
I don't believe this will work so easily. As said in the man page: "In 
general this will degrade performance"

Direct means it goes directly to the disk. When you record 2 or more 
streams, or want playback while recording (time shift), then the disc 
heads will jump between the positions for every read/wrtie access 
(synchronous, unbuffered). To get performance you need to implement a 
smart readahead variant for playback and a "huge" buffer for recording, 
so it has a lot of data collected before a write access is done.

When I look at my HDD LED I see that it is off for long periods, even 
when recording/playing multiple streams. This is quite effective and 
gives a good performance, if it would just free the cache buffers 
afterwards...

So I think a kind of I/O subsystem must be implemented that suites our 
needs, i.e.
Playback:
- random access to every octet in the stream translating it to the 
required buffer(s),
- a limited cache of a few minutes or less (sometimes you skip back to 
replay a szene, e.g. when a phone call interrupts you, or you want to 
enjoy the last goal of your favorite soccer team in slow motion ;-)).
- a "big" readahead buffer that minimizes the read accesses while 
fetching "new" data
Record:
- a minimum write-section size. Probably we would need a bigger ring 
buffer for this (?)

Any more requirements?

Andreas



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