Re: [PATCHv4 18/43] block: define BIO_MAX_PAGES to HPAGE_PMD_NR if huge page cache enabled

[Ming Lei]

On Wed, Oct 26, 2016 at 12:13 PM, Andreas Dilger <adilger@xxxxxxxxx> wrote:
> On Oct 25, 2016, at 6:54 AM, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> 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.

I am preparing for the multipage bvec support[1], and once it is ready the
default 256 bvecs should be enough for normal cases.

I will post them out next month for review.

[1] https://github.com/ming1/linux/tree/mp-bvec-0.1-v4.9

Thanks,
Ming

>
> Cheers, Andreas
>
>
>
>
>



-- 
Ming Lei