Re: [PATCH] ext4: Use single thread to perform DIO unwritten convertion

[Mingming Cao <cmm@us.ibm.com>]

On Sat, 2011-03-05 at 12:46 -0500, Ted Ts'o wrote: 
> On Thu, Mar 03, 2011 at 11:29:54AM -0800, Mingming Cao wrote:
> > While running ext4 testing on multiple core, we found there are per
> > cpu ext4-dio-unwritten threads processing conversion from unwritten
> > extents to written for IOs completed from async direct IO patch.
> > Per filesystem is enough, we don't need per cpu threads to work on
> > conversion.
> > 
> > Signed-off-by: Mingming Cao <cmm@us.ibm.com>
> 
> Eric, would you be able to do a very quick sanity check on your
> 48-core machine?  I can definitely see how having a huge number of
> threads per file system could be problematic, especially on a system
> with 32 or 64 ext4 file systems.  I'm curious though if we'll end up
> taking a performance hit on direct I/O workloads.
> 
> If I remember correctly we currently have large file create with DIO
> turned off, right?  Would it be possible to do a large file create
> with DIO enabled, and do a quick run both with and without this patch?
> 
The background thread performs the conversion when IOs from async dio
writing to holes/preallocated is completed. So would need to setup
fallocated files and running async and direct IO would possible to
exercise any potential scalability issue with the background dio
conversion thread... 

I took a look at FFSB, it doesn't support fallocate and async IO yet.
But fio does support aio and fallocate. This is a simple fio profile I
use for test file being setup by fallocate() and run random aio dio over
it. See it is useful for Eric to give it a try or a reference on his 48
core.

examples$ cat aio-setup 
; Random read/write to fallocat files with aio dio
[global]
ioengine=libaio
direct=1
rw=randrw
bs=4k
size=2m
filesize=1024m
fallocate=1
directory=/tmp

[file1]
iodepth=4

> In the future it would also be interesting to see how we are doing
> versus other file systems using a DIO workload.  This is a probably
> another area where I suspect some lockstat and oprofile runs may give
> us opportunities for further optimization.
> 
>        	     	  	  	      - Ted
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