[vdr] [PATCH] UnbufferedFile improvements v2

Ralf Müller rmvdr at bj-ig.de
Sun Nov 20 15:53:19 CET 2005

Artur Skawina schrieb:

> well, vdr w/ the recent cUnbufferedFile changes was flushing the data
> buffers in huge burst; this was even worse than slowly filling up the
> caches -- the large (IIRC ~10M) bursts caused latency problems (apps
> visibly freezing etc).

Does this freezing apply to local disk access or only to network
filesystems. My personal VDR is a dedicated to VDR usage system which
uses a local hard disk for storage. So I don't have applications
parallel to vdr which can freeze nor I can actually test behaviour on
network devices. Seems you have both of this extra features so it would
be nice to know more about this.

For local usage I found that IO interruptions of less then a second (10
MB burst writes on disks which give a hell lot more then 10MB/sec) have
no negative side effects. But I can imagine that on 10Mbit ethernet it
could be hard to have these bursts ... I did not think about this when
writing the initial patch ...

> This patch makes vdr use a much more aggressive disk access strategy.
> Writes are flushed out almost immediately and the IO is more evenly
> distributed. While recording and/or replaying the caches do not grow and
> when vdr is done accessing a video file all cached data from that file
> is dropped.

Actually with the patch you attached my cache _does_ grow. It does not
only grow - it displaces the inode cache, to avoid this the initial
patch has been created. To make it worse - when cutting a recording and
have the newly cut recording replayed at the same time I have major
hangs in replay.

I had a look at your patch - it looked very well. But for whatever
reason it doesn't do what it is supposed to do at my VDR. I currently
don't know why it doesn't work here for replay - the code there looked good.

I like the heuristics you used to deal with read ahead - but maybe these
lead to the leaks I experience here. I will have a look at it. Maybe I
can find out something about it ...

> I've tested this w/ both local disks and NFS mounted ones, and it seems
> to do the right thing. Writes get flushed every 1..2s at a rate of
> .5..1M/s instead of the >10M bursts. 

To be honest - I did not found the place where writes get flushed in
your patch. posix_fadvise() doesn't seem to influence flushing at all.
It only applies to already written buffers. So the normal write
strategie is used with your patch - collect data until the kernel
decides to write it to disk. This leads to "collect about 300MB" here
and have an up to 300MB burst then. This is a bit more heavy then the
10MB bursts before ;)


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