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Subject: Re: [PATCH v1 2/6] fs: use on-stack-bio if backing device has
 BDI_CAP_SYNC capability
To: Christoph Hellwig <hch@lst.de>,
        Dan Williams <dan.j.williams@intel.com>
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 <20170811104615.GA14397@lst.de>
From: Jens Axboe <axboe@kernel.dk>
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Date: Fri, 11 Aug 2017 08:26:59 -0600
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On 08/11/2017 04:46 AM, Christoph Hellwig wrote:
> On Wed, Aug 09, 2017 at 08:06:24PM -0700, Dan Williams wrote:
>> I like it, but do you think we should switch to sbvec[<constant>] to
>> preclude pathological cases where nr_pages is large?
> 
> Yes, please.
> 
> Then I'd like to see that the on-stack bio even matters for
> mpage_readpage / mpage_writepage.  Compared to all the buffer head
> overhead the bio allocation should not actually matter in practice.

I'm skeptical for that path, too. I also wonder how far we could go
with just doing a per-cpu bio recycling facility, to reduce the cost
of having to allocate a bio. The on-stack bio parts are fine for
simple use case, where simple means that the patch just special
cases the allocation, and doesn't have to change much else.

I had a patch for bio recycling and batched freeing a year or two
ago, I'll see if I can find and resurrect it.

-- 
Jens Axboe