Subject: Re: [Fwd: [linux-audio-dev] info point on linux hdr]
From: Stephen C. Tweedie (sct_AT_redhat.com)
Date: Mon Apr 17 2000 - 18:40:37 EEST
Hi,
On Mon, Apr 17, 2000 at 04:50:05PM +0200, Benno Senoner wrote:
>
> Stephen, I tried all possible combinations , in my hdrbench code.
...
> I tried:
> -fsync() on all write descriptors at regular intervals ranging from 1sec to
> 10sec
> - fdatasync() on all write descriptors , same as above
> - sync()
> - opening output files with O_SYNC
>
> NO LUCK AT ALL. :-(
Of course not. You are doing sync IO in each case, and are doing it
from a single thread.
> Again I think O_DIRECT is the only way which will allow us to use smaller
> buffers, but we will gain almost nothing in terms of total delivered MB/secs.
No. O_DIRECT will gain you *nothing* in terms of IO scheduling over O_SYNC.
The only thing O_DIRECT gains is zero-copy and zero-caching, which results
in lower CPU utilisation.
The only way you can get much better is to do non-writeback IO
asynchronously. Use O_SYNC for writes, and submit the IOs from multiple
threads, to let the kernel schedule the multiple IOs. Use large block
sizes for each IO to prevent massive amounts of disk seeking. O_DIRECT
in this case is not an instant win: it is completely orthogonal to the
IO scheduling issue.
--Stephen
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