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From: Jens Axboe <axboe@kernel.dk>
To: Matthew Wilcox <willy@infradead.org>
Cc: linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
	hch@infradead.org, clm@fb.com
Subject: Re: [PATCHSET 0/3] io_uring: add sync_file_range and drains
Date: Thu, 11 Apr 2019 09:23:12 -0600	[thread overview]
Message-ID: <d7d45594-d1bb-493c-6e92-9475e6ffbe7c@kernel.dk> (raw)
In-Reply-To: <20190411151657.GD22763@bombadil.infradead.org>

On 4/11/19 9:16 AM, Matthew Wilcox wrote:
> On Thu, Apr 11, 2019 at 09:06:54AM -0600, Jens Axboe wrote:
>> In continuation of the fsync barrier patch from the other day, I
>> reworked that patch to turn it into a general primitive instead. This
>> means that any command can be flagged with IOSQE_IO_DRAIN, which will
>> insert a sequence point in the queue. If a request is marked with
>> IOSQE_IO_DRAIN, then previous commands must complete before this one
>> is issued. Subsequent requests are not started until the drain has
>> completed. The latter is a necessity since we track this through the
>> CQ index. If we allow later commands, then they could complete before
>> earlier commands and we'd mistakenly think that we have satisfied the
>> sequence point.
> 
> That's potentially going to cause quite the bubble in the pipeline of
> commands being sent.

Definitely.

> Do consumers know which writes they are going to want to fence?  We could
> do something like tag each command with a stream ID and then fence a
> particular stream.  We'd need one nr_pending counter per stream, but
> that should be pretty cheap.

Or you could just split your streams between io_urings. That has other
overhead of course in terms of resources, but it'd avoid having to do
any extra accounting on the kernel side. A pending counter is not
necessarily cheap, though it'd be acceptable if we required writes
that you want to fence to be tagged (hence it wouldn't happen for
"normal" IO).

-- 
Jens Axboe


  reply	other threads:[~2019-04-11 15:23 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-11 15:06 [PATCHSET 0/3] io_uring: add sync_file_range and drains Jens Axboe
2019-04-11 15:06 ` [PATCH 1/3] io_uring: add support for marking commands as draining Jens Axboe
2019-05-06 18:03   ` Andres Freund
2019-04-11 15:06 ` [PATCH 2/3] fs: add sync_file_range() helper Jens Axboe
2019-04-11 15:06 ` [PATCH 3/3] io_uring: add support for IORING_OP_SYNC_FILE_RANGE Jens Axboe
2019-04-11 15:16 ` [PATCHSET 0/3] io_uring: add sync_file_range and drains Matthew Wilcox
2019-04-11 15:23   ` Jens Axboe [this message]
2019-04-11 16:19   ` Chris Mason

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