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List:       linux-mm
Subject:    Re: [PATCHv4 18/43] block: define BIO_MAX_PAGES to HPAGE_PMD_NR if huge page cache enabled
From:       Andreas Dilger <adilger () dilger ! ca>
Date:       2016-10-26 4:13:13
Message-ID: BD27B76A-AF34-48B9-8D4F-F69AD2C17C66 () dilger ! ca
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On Oct 25, 2016, at 6:54 AM, Kirill A. Shutemov <kirill@shutemov.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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