Re: [PATCH 07/13] aio: enabled thread based async fsync

[Benjamin LaHaise <bcrl@kvack.org>]

On Thu, Jan 21, 2016 at 08:45:46AM +1100, Dave Chinner wrote:
> Filesystems *must take locks* in the IO path. We have to serialise
> against truncate and other operations at some point in the IO path
> (e.g. block mapping vs concurrent allocation and/or removal), and
> that can only be done sanely with sleeping locks.  There is no way
> of knowing in advance if we are going to block, and so either we
> always use threads for IO submission or we accept that occasionally
> the AIO submission will block.

I never said we don't take locks.  Still, we can be more intelligent 
about when and where we do so.  With the nonblocking pread() and pwrite() 
changes being proposed elsewhere, we can do the part of the I/O that 
doesn't block in the submitter, which is a huge win when possible.

As it stands today, *every* buffered write takes i_mutex immediately 
on entering ->write().  That one issue alone accounts for a nearly 10x 
performance difference between an O_SYNC write and an O_DIRECT write, 
and using O_SYNC writes is a legitimate use-case for users who want 
caching of data by the kernel (duplicating that functionality is a huge 
amount of work for an application, plus if you want the cache to be 
persistent between runs of an app, you have to get the kernel to do it).

		-ben

> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com

-- 
"Thought is the essence of where you are now."

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