linux-kernel - Re: [PATCH v1 2/6] fs: use on-stack-bio if backing device has BDI_CAP

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Message-ID: <20c5b30a-b787-1f46-f997-7542a87033f8@kernel.dk>
Date:   Fri, 11 Aug 2017 08:26:59 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        "karam . lee" <karam.lee@....com>, seungho1.park@....com,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
        Vishal Verma <vishal.l.verma@...el.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        kernel-team <kernel-team@....com>
Subject: Re: [PATCH v1 2/6] fs: use on-stack-bio if backing device has
 BDI_CAP_SYNC capability

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