Date:	Fri, 4 Dec 2015 00:43:07 +0530
From:	lokesh jaliminche <lokesh.jaliminche@...il.com>
To:	Andreas Dilger <adilger@...ger.ca>, linux-ext4@...r.kernel.org
Subject: Re: Regarding random grouop search start for allocation of inode.

Ohh thanks for the clarification. There is one more thing I would like
to point out here.
In the code there  is a loop to scan the groups for inode
alllocation(Inside find_group_orlov function).
There are some policies for group selection . while scanning the
groups, it checks for these
policies to be satisfied.
If a particular group satisfy these properties it should get selected
for inode allocation but instead
it does further lookup in next groups.
I think there is missing breaking condition. I have added break over
there and here is the
patch for that. Any reason for not having break condition over here ?

diff -Nur linux-2.6.32-431.17.1.el6.x86_64/fs/ext4/ialloc.c
linux-2.6.32-431.17.1.el6.x86_64/fs/ext4/ialloc.c
--- linux-2.6.32-431.17.1.el6.x86_64/fs/ext4/ialloc.c    2014-04-12
01:20:31.000000000 +0530
+++ linux-2.6.32-431.17.1.el6.x86_64/fs/ext4/ialloc.c    2015-11-29
21:36:51.805542209 +0530
@@ -529,6 +529,7 @@
             grp = g;
             ret = 0;
             best_ndir = stats.used_dirs;
+            break;
         }
         if (ret)
         goto fallback;

Thanks & Regards,
   Lokesh



On Thu, Dec 3, 2015 at 11:28 PM, Andreas Dilger <adilger@...ger.ca> wrote:
> On Dec 3, 2015, at 01:07, lokesh jaliminche <lokesh.jaliminche@...il.com> wrote:
>>
>> 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
>
> Starting the inode search at the beginning of the disk each time
> means that inode allocation will be inefficient because it will search
> over groups that are mostly or entirely full already.
>
> Allocating the new directory in a semi-random group, one that is
> relatively unused, ensures that new
> inode and block allocations are relatively efficient afterward.
>
> Cheers, Andreas
>
>> 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:
>>> ==========
>>> В·В·В·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
>> N‹§Іжмrё›yъиљШbІX¬¶З§vШ^–)Ює{.nЗ+‰·ҐЉ{±{ xЉ{ayє К‡Ъ™л,j ­ўfЈў·hљ‹аz№ ®wҐўё ў·¦j:+v‰ЁЉwиjШm¶џяѕ «‘кзzZ+ѓщљЋЉЭўj"ќъ!¶i

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