Re: kernel 2.6.13 buffer strangeness

[Nate Diller]

On 9/9/05, Anthony Wesley <awesley@xxxxxxxxxxxxxxx> wrote:
> Okay, just tested a couple of things, here's what I see...
> 
> I tested the write speed to the usb2 hard disk and got 21MBytes/sec. It's a
> laptop 
> hard drive, only 5400rpm so this is not surprising.
> 
> I did this test with my video capture app running, but just displaying data
> and not writing,
> 
> I have another laptop usb2 hard drive here which I just tested and got
> 17MBytes/sec - it's
> a 4200rpm drive so again not surprising numbers.
> 
> The video data is written as individual frames, so the efficiency is a bit
> below the raw throughput,
> but my tests were transferring 1.5Gb of data as raw frames - exactly the
> same way that Coriander would
> write its data.
> 
> I'd bet a large sum of money that these hard disk figures are correct to
> within a few percent.
> 
> Also, when I am actually capturing I have timed by stopwatch how long the
> disk activity light
> is on, and reached about the same conclusion.
> 
> Working the problem from the other direction, the only way to explain the
> early throttling that
> I see would be if *almost no data* is being written to the disk, and this is
> plainly not the case. Even if
> the disk were running at a greatly reduced rate of (say) 10MBytes/sec I
> would still see 86 seconds
> of buffering before the throttle kicks in, and so far I have managed only to
> get to about 65 or 70.
> 
> regards, Anthony
> 
i need a vmstat trace ('vmstat 1 > logfile', then run the test, then
^c)  before i can comment further.  also, your filesystem and
scheduler would be interesting to know.  you can find out the
scheduler with 'cat /sys/block/[drive]/queue/scheduler'.

NATE

> Nate Diller wrote:
> 
> 
> >>Setting dirty_ratio and dirty_background_ratio to 90/10 puts me back at
> >>around 50 seconds, i.e. where I started.
> >>
> > 
> > this setting should do the trick, so there's something going on here
> > that isn't expected.
> > 
> > 
> >>So as far as I can see there is *no way* to get 3 minutes worth of
> buffering
> >>by adjusting these parameters.
> >>
> >>Just to remind everyone - I have video data coming in at 25MBytes/sec and
> I
> >>am writing it out to a usb2 hard disk that can sustain 17MBytes/sec. I
> want
> >>my video capture to run at full speed as long as possible by having the
> >>7MBytes/sec deficit slowly eat up the available RAM in the machine. I
> have
> >>1.5Gb of RAM, 1.3Gb available for buffers, so this should take 3 minutes
> to
> >>consume at 7MBytes/sec.
> >>
> >>So, I've tried all the combinations on dirty_ratio and
> >>dirty_background_ratio and they *do not help*.
> >>
> > 
> > dirty_ratio is the tubable you want, if it's not working correctly,
> > either there's a problem with your setup, or a bug
> > 
> > 
> >>Can anyone suggest something else that I might try? The goal is to have
> >>25MBytes/sec streaming video for about 3 minutes. 
> >>
> > 
> > how sure are you that you're getting 17MB/s during this test?  can you
> > run "vmstat 1" while this is running to verify?  which FS and
> > scheduler?
> > 
> > just for interest, what's the raw disk bandwidth (use hdparm, or run a
> > dd, or something)?  it would obviously be much better to sustain
> > 25MB/s to disk
> > 
> > 
> >>Or is this simply not possible with the current kernel I/O setup? Do I
> have
> >>to do something elaborate myself, like build a big RAM buffer, mount the
> >>disk synchronous, do the buffering myself in userland...??
> >>
> > 
> > this should be possible, although it could be considered a bit risky WRT
> OOM.
> > 
> > NATE
> 
> -- 
> Anthony Wesley
> Director and IT/Network Consultant
> Smart Networks Pty Ltd
> Acquerra Pty Ltd
> 
> Anthony.Wesley@xxxxxxxxxxxxxxx
> Phone: (02) 62595404 or 0419409836
>
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