Date:	Thu, 17 Dec 2015 20:04:40 +0530
From:	lokesh jaliminche <lokesh.jaliminche@...il.com>
To:	Andreas Dilger <adilger@...ger.ca>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: Regarding random grouop search start for allocation of inode.

Hey thanks for that, I will push a patch for this ASAP :-)

Regards,
Lokesh

On Wed, Dec 16, 2015 at 3:02 AM, Andreas Dilger <adilger@...ger.ca> wrote:
> On Dec 15, 2015, at 3:33 AM, lokesh jaliminche <lokesh.jaliminche@...il.com> wrote:
>>
>> "No, this isn't correct.  This loop is looking for the *best* group it
>> can find, and your "break" would have it exit the loop as soon as the
>> *first* group that matches the conditions is found."
>>
>> But as we are checking  all the groups with the same conditions then
>> how it guarantees better group selection ? As per my understanding  we
>> are just wasting time in looping.
>
> The important part of the loop is ensuring that the selected group is always
> improving over the previous one:
>
>                         if (le16_to_cpu(desc->bg_used_dirs_count) >= best_ndir)
>                                 continue;
>
> The "best_ndir" value tracks for the best group found so far the number of
> used directories in the group, and if the new directory has fewer directories
> than the  previous "best" directory and still meets all the other criteria
> (fewer than average inodes allocated, etc) then the new group will be chosen.
>
> That said, you are correct that the loop can spend a lot of time searching
> needlessly.  It would be trivial to add a check at the start of the loop:
>
>                         /* the group can't get any better than empty */
>                         if (desc->bg_free_inodes_count == inodes_per_group &&
>                             desc->bg_free_blocks_count ==
>                                                 EXT4_BLOCKS_PER_GROUP(sb))
>                                 goto found;
>
> This jumps directly to "found" with "desc" and "group" already set, so
> there is no need to go through the extra steps of setting "best_desc" and
> "best_group" and then break out of the loop just to set "desc" and "group"
> again.
>
> Since you are the one to find this optimization, could you please submit a
> patch to this effect so you get the credit.
>
> Cheers, Andreas
>
>> On Sat, Dec 5, 2015 at 3:24 AM, Andreas Dilger <adilger@...ger.ca> wrote:
>>>
>>>> On Dec 3, 2015, at 12:13 PM, lokesh jaliminche <lokesh.jaliminche@...il.com> wrote:
>>>>
>>>> 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;
>>>
>>> No, this isn't correct.  This loop is looking for the *best* group it can find,
>>> and your "break" would have it exit the loop as soon as the *first* directory
>>> that matches the conditions is found.  Since those conditions are fairly weak,
>>> for example that the group actually has free inodes, and it has better than
>>> average free inodes and blocks, it makes sense to search beyond just the first
>>> matching group.
>>>
>>> That said, it also doesn't make sense to search beyond a "perfect" group that
>>> has no allocated inodes and no allocated blocks, so a break condition could be
>>> added to this loop and make it more efficient, especially for very large
>>> filesystems that have 128k+ groups.
>>>
>>> It should be noted that this part of the algorithm only applies to "top level"
>>> directories (those below the root inode, or with the EXT4_INODE_TOPDIR flag
>>> set, so searching a bit longer for a good group is not a bad idea in this case.
>>>
>>> Cheers, Andreas.
>>>
>>>> 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
>>>
>>>
>>> Cheers, Andreas
>>>
>>>
>>>
>>>
>>>
>
>
> Cheers, Andreas
>
>
>
>
>

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