Re: write-behind on streaming writes

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Jun 06, 2012 at 08:14:08AM -0400, Vivek Goyal wrote:

[..]
> I think it is happening because sync_file_range() will send all
> the writes as SYNC and it will compete with firefox IO. On the other
> hand, flusher's IO will show up as ASYNC and CFQ  will be penalize it
> heavily and firefox's IO will be prioritized. And this effect should
> just get worse as more processes do sync_file_range().

Ok, this time I tried the same test again but with 4 processes doing
writes in parallel on 4 different files.

And with sync_file_range() things turned ugly. Interactivity was very poor. 

firefox launch test took around 1m:45s with sync_file range() while it
took only about 35seconds with regular flusher threads.

So sending writeback IO synchronously wreaks havoc.

Thanks
Vivek

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