linux-ext4 - Re: Regarding random grouop search start for allocation of inode.

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Date:	Thu, 3 Dec 2015 13:37:34 +0530
From:	lokesh jaliminche <lokesh.jaliminche@...il.com>
To:	linux-ext4@...r.kernel.org
Subject: Re: Regarding random grouop search start for allocation of inode.

Thought of giving more clarification on my question
why group search start is random ? because we can also start  search
for valid groups for inode allocation from the start . As this group
search is random  inode selection might go to end of groups which
might affect IO performance

On Thu, Dec 3, 2015 at 1:14 PM, lokesh jaliminche
<lokesh.jaliminche@...il.com> wrote:
> hello folks,
>                 I am new to ext4 code. I was going through the
> ext4-source for allocation of inode.
> There is one thing that I did not understand while selection of groups
> for inode allocation . I came across this code snippet which is part
> of find_group_orlov function. question is, why group search start is
> random ?
>
> Code snippet:
> ==========
> ···if (qstr) {
> »·······»·······»·······hinfo.hash_version = LDISKFS_DX_HASH_HALF_MD4;
> »·······»·······»·······hinfo.seed = sbi->s_hash_seed;
> »·······»·······»·······ldiskfsfs_dirhash(qstr->name, qstr->len, &hinfo);
> »·······»·······»·······grp = hinfo.hash;
> »·······»·······} else
> »·······»·······»·······get_random_bytes(&grp, sizeof(grp));
> »·······»·······parent_group = (unsigned)grp % ngroups;
> »·······»·······for (i = 0; i < ngroups; i++) {
> »·······»·······»·······g = (parent_group + i) % ngroups;
> »·······»·······»·······get_orlov_stats(sb, g, flex_size, &stats);
> »·······»·······»·······if (!stats.free_inodes)
> »·······»·······»·······»·······continue;
> »·······»·······»·······if (stats.used_dirs >= best_ndir)
> »·······»·······»·······»·······continue;
> »·······»·······»·······if (stats.free_inodes < avefreei)
> »·······»·······»·······»·······continue;
> »·······»·······»·······if (stats.free_blocks < avefreeb)
> »·······»·······»·······»·······continue;
> »·······»·······»·······grp = g;
> »·······»·······»·······ret = 0;
> »·······»·······»·······best_ndir = stats.used_dirs;
> »·······»·······}
>
> Thanks & Regards,
>   Lokesh