linux-ext4 - Re: [PATCHv4 18/43] block: define BIO_MAX_PAGES to HPAGE_PMD

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Message-Id: <BD27B76A-AF34-48B9-8D4F-F69AD2C17C66@dilger.ca>
Date:   Tue, 25 Oct 2016 22:13:13 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jan Kara <jack@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-block@...r.kernel.org
Subject: Re: [PATCHv4 18/43] block: define BIO_MAX_PAGES to HPAGE_PMD_NR if huge page cache enabled

On Oct 25, 2016, at 6:54 AM, Kirill A. Shutemov <kirill@...temov.name> wrote:
> 
> On Tue, Oct 25, 2016 at 12:21:22AM -0700, Christoph Hellwig wrote:
>> On Tue, Oct 25, 2016 at 03:13:17AM +0300, Kirill A. Shutemov wrote:
>>> We are going to do IO a huge page a time. So we need BIO_MAX_PAGES to be
>>> at least HPAGE_PMD_NR. For x86-64, it's 512 pages.
>> 
>> NAK.  The maximum bio size should not depend on an obscure vm config,
>> please send a standalone patch increasing the size to the block list,
>> with a much long explanation.  Also you can't simply increase the size
>> of the largers pool, we'll probably need more pools instead, or maybe
>> even implement a similar chaining scheme as we do for struct
>> scatterlist.
> 
> The size of required pool depends on architecture: different architectures
> has different (huge page size)/(base page size).
> 
> Would it be okay if I add one more pool with size equal to HPAGE_PMD_NR,
> if it's bigger than than BIO_MAX_PAGES and huge pages are enabled?

Why wouldn't you have all the pool sizes in between?  Definitely 1MB has
been too small already for high-bandwidth IO.  I wouldn't mind BIOs up to
4MB or larger since most high-end RAID hardware does best with 4MB IOs.

Cheers, Andreas






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