[mythtv-users] LiveTV zapping speed

Brian J. Murrell brian at interlinx.bc.ca
Mon Sep 5 14:08:08 UTC 2011

On 11-09-04 07:33 PM, Gavin Hurlbut wrote:
> Your whole message seems to be predicated on a false assumption.

What assumption is that?

> The only
> way that you can extensively use "the cache" in this way is if your frontend
> and backend are the same machine.

Not at all.  I don't have any BE/FE machines here so why would that
use-case be my assumption?  You don't have to have a FE on the BE
machine to take advantage of the cached pages.  Whether the FE is remote
or local, the caching of pages is still going to have the efficiency of
not having to read them from disk to deliver them to the client.

> MythTV is designed to allow that, but it
> also is designed for the backend(s) and frontend(s) to be totally different
> machines.

Don't I know it.  All of my FEs here are remote.

> In this eventuality, there's no way to share a cache as the
> recording and the playback are separated by a network connection.

I never implied that the cache was shared between the FE and BE with any
closer than a network socket.  My point was that if the FE did small
reads so as to not require a multi-second lag (to avoid starvation), at
least those small reads would not translate into small disk accesses
(because they would come from the server's page cache), which is of
course horrible for disk I/O -- which is for probably most people the
slowest link in the chain.

> Yes, the reads by the frontends are likely reading from cache in the
> backend,


> but you can't synchronize the read/writes in the way that you seem
> to be suggesting.

I'm not suggesting any synchronization.  I'm merely suggesting doing
smaller reads so that the "lag buffer" need not be so large -- when
necessary.  Of course, if the user pauses livetv, you can increase the
size of your reads as the "lag buffer" will be larger.


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