linux-kernel - Re: [PATCH 0/4] Page based O

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Date:	Wed, 19 Aug 2009 15:44:06 +0300
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Jens Axboe <jens.axboe@...cle.com>
CC:	linux-kernel@...r.kernel.org, zach.brown@...cle.com,
	hch@...radead.org
Subject: Re: [PATCH 0/4] Page based O_DIRECT v2

On 08/18/2009 11:34 AM, Jens Axboe wrote:
> Hi,
> 
> Updated patchset for page based O_DIRECT. I didn't include the
> loop bits this time, lets focus on getting these core bits into
> shape and then loop is easily patchable on top of this.
> 
> Changes since last post:
> 
> - Changed do_dio() to generic_file_direct_IO() as per Christophs
>   suggestion.
> - Split the first patch into two parts. One simply adds dio_args
>   and maintains the current code, the next has the functional change
>   but without changing file systems (except NFS).
> - Add ->rw to dio_args (Christoph).
> - A locking fixup. Not really related, but should be fixed up anyways.
> 
> There are at least two pending things to work on:
> 
> 1) NFS is still broken, I get a crash in freeing some data that
>    is not related to the pages. Will debug this.
> 2) As Christoph suggested, we need some way to wait for a dio
>    when all segments are submitted. Currently it waits for each
>    segment. Not sure how best to solve this issue, will think a
>    bit more about this. Basically we need to pass down the wait
>    list to the generic_file_direct_IO() and have that do the
>    queue kick and wait.
> 

Jens hi.

I please have some basic question on the subject?

[1]
So before, the complete iovec from user mode could potentially be
submitted in a single request, depending on the implementor.
With new code, each iovec entry is broken to it's few pages and
is submitted as a separate request. This might not be bad for
block based devices that could see these segments merged back by the
IO elevator. But what about the other implementers that see a
grate performance boost in the current scatter-gather nature of the
iovec API. It's almost as if the application was calling the kernel
for each segment separately.

I wish you would use a more generic page carrier then page-* array.
and submit the complete iovec at once.

We used to use scatter-lists but these are best only used inside DMA
engines and Drivers as they are more then 2 times too big. The ideal for
me is the bio_vec array as used inside a bio. scatter-list has all these
helpers, iterators, and wrappers, which bio_vec do not, so I don't know
what the best choice is.

But your current solution, (from inspection only I have not tested any of
this), might mean a grate performance degradation for some work scenarios.
For example a user-mode app the gathers lots of small memory sources and
hopes to write it as a single very large on-the-wire-NFS-write , might find
itself writing lots of small on-the-wire-NFS-writes.

[2]
Please address linux-fsdevel on these patches. lkml is so crowded and after
all these files do sit in fs/

Thanks
Boaz
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