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Date:   Thu, 25 Feb 2021 20:43:20 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     harshad shirwadkar <harshadshirwadkar@...il.com>
Cc:     Благодаренко Артём 
        <artem.blagodarenko@...il.com>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Alex Zhuravlev <bzzz@...mcloud.com>,
        Shuichi Ihara <sihara@....com>
Subject: Re: [PATCH v2 4/5] ext4: improve cr 0 / cr 1 group scanning

On Feb 21, 2021, at 8:59 PM, harshad shirwadkar <harshadshirwadkar@...il.com> wrote:
> 
> Thank you for all the feedback Andreas and Artem. Some comments below:

Thank you for working on this.  It is definitely an area that can use
this improvement.

>> I was wondering about the cost of the list/tree maintenance as well,
>> especially since there was a post from "kernel test robot" that this
>> patch introduced a performance regression.
> 
> Yeah, I'm pretty sure that the kernel test robot is complaining mainly
> because of list/tree maintenance. I think if this optimization is
> turned off, we probably should not even maintain the tree / lists.
> That has one downside that is that we will have to disallow setting
> this option during remount, which I guess is okay?

I think it is reasonable to not be able to change this at runtime.
This would only make a difference for a limited number of testers,
and virtually all users will never know it exists at all.

>> It would also make sense for totally full groups to be kept out of the
>> rb tree entirely, since they do not provide any value in that case (the
>> full groups will never be selected for allocations), and they just add
>> to the tree depth and potentially cause an imbalance if there are many
>> of them.  That also has the benefit of the rbtree efficiency *improving*
>> as the filesystem gets more full, which is right when it is most needed.
> 
> Ack
> 
>> It might also make sense to keep totally empty groups out of the rbtree,
>> since they should always be found in cr0 already if the allocation is
>> large enough to fill the whole group?  Having a smaller rbtree makes
>> every insertion/removal that much more efficient.
> 
> Ack
> 
>> Those groups will naturally be re-added into the rbtree when they have
>> blocks freed or allocated, so not much added complexity.
>> 
>> 
>> Does it make sense to disable "mb_optimize_scan" if filesystems are
>> smaller than a certain threshold?  Clearly, if there are only 1-2
>> groups, maintaining a list and rbtree has no real value, and with
>> only a handful of groups (< 16?) linear searching is probably as fast
>> or faster than maintaining the two data structures.  That is similar
>> to e.g. bubble sort vs. quicksort, where it is more efficient to sort
>> a list of ~5-8 entries with a dumb/fast algorithm instead of a complex
>> algorithm that is more efficient at larger scales.  That would also
>> (likely) quiet the kernel test robot, if we think that its testing is
>> not representative of real-world usage.
> 
> Ack, these are good optimizations. I'll add these in V3.

For testing purposes it should be possible to have "mb_optimize_scan=1"
force the use of this option, even if the filesystem is small.

> Besides the optimizations mentioned here, I also think we should add
> "mb_optimize_linear_limit" or such sysfs tunable which will control
> how many groups should mballoc search linearly before using tree /
> lists for allocation? That would help us with the disk seek time
> performance.

There is already a linear search threshold parameters for mballoc,
namely mb_min_to_scan and mb_max_to_scan that could be used for this.
I think we could use "mb_min_to_scan=10" (the current default), or
maybe shrink this a bit (4?) if "mb_optimize_scan" is enabled.

> We discussed on our last call that we probably should consult with the
> block device's request queue to check if the underlying block device
> is rotational or not. However, we also discussed that for more complex
> devices (such as DMs setup on top of HDD and SSD etc), whether the
> device is rotational or not is not a binary answer and we would need a
> more complex interface (such as logical range to "is_rotational"
> table) to make intelligent choice in the file system. Also, in such
> cases, it is not clear if such a table needs to be passed to the file
> system during mkfs time? or at mount time? or at run time?

I don't think the hybrid case is very important yet.  By far the
most common case is going to be "rotational=1" or "rotational=0"
for the whole device, so we should start by only optimizing for
those cases.  DM looks like it returns "rotational=0" correctly
when a composite device it is made of entirely non-rotational
devices and "rotational=1" as it should when it is a hybrid
HDD/SSD device (which I have in my local system).

> Given the number of unknowns in the above discussion, I propose that
> we start simple and evolve later. So my proposal is that we add a
> "mb_optimize_linear_limit" tunable that accepts an integer value. In
> the kernel, for non-rotational devices, that value will be defaulted
> to 0 (which means no linear scan) and for rotational devices, that
> value will be defaulted to a reasonable value (-- not sure what that
> value would be though - 4?). This default can be overridden using the
> sysfs interface. We can later evolve this interface to accept more
> complex input such as logical range to rotational status.
> 
> Does that sound reasonable?

Yes, modulo using the existing "mb_min_to_scan" parameter for this.
I think 4 or 8 or 10 groups is reasonable (512MB, 1GB, 1.25GB),
since if it needs a seek anyway then we may as well find a good
group for this.

Cheers, Andreas

> 
>> 
>>> On Feb 11, 2021, at 3:30 AM, Andreas Dilger <adilger@...ger.ca> wrote:
>> 
>>>> This function would be more efficient to do the list move under a single
>>>> write lock if the order doesn't change.  The order loop would just
>>>> save the largest free order, then grab the write lock, do the list_del(),
>>>> set bb_largest_free_order, and list_add_tail():
>>>> 
>>>> mb_set_largest_free_order(struct super_block *sb, struct ext4_group_info *grp)
>>>> {
>>>>     struct ext4_sb_info *sbi = EXT4_SB(sb);
>>>>     int i, new_order = -1;
>>>> 
>>>>     for (i = MB_NUM_ORDERS(sb) - 1; i >= 0; i--) {
>>>>             if (grp->bb_counters[i] > 0) {
>>>>                     new_order = i;
>>>>                     break;
>>>>             }
>>>>     }
>>>>     if (test_opt2(sb, MB_OPTIMIZE_SCAN) && grp->bb_largest_free_order >= 0) {
>>>>             write_lock(&sbi->s_mb_largest_free_orders_locks[
>>>>                                           grp->bb_largest_free_order]);
>>>>             list_del_init(&grp->bb_largest_free_order_node);
>>>> 
>>>>             if (new_order != grp->bb_largest_free_order) {
>>>>                     write_unlock(&sbi->s_mb_largest_free_orders_locks[
>>>>                                           grp->bb_largest_free_order]);
>>>>                     grp->bb_largest_free_order = new_order;
>>>>                     write_lock(&sbi->s_mb_largest_free_orders_locks[
>>>>                                           grp->bb_largest_free_order]);
>>>>             }
>>>>             list_add_tail(&grp->bb_largest_free_order_node,
>>>>                   &sbi->s_mb_largest_free_orders[grp->bb_largest_free_order]);
>>>>             write_unlock(&sbi->s_mb_largest_free_orders_locks[
>>>>                                           grp->bb_largest_free_order]);
>>>>     }
>>>> }
>> 
>> In looking at my previous comment, I wonder if we could further reduce
>> the list locking here by not moving an entry to the end of the *same*
>> list if it is not currently at the head?  Since it was (presumably)
>> just moved to the end of the list by a recent allocation, it is very
>> likely that some other group will be chosen from the list head, so
>> moving within the list to maintain strict LRU is probably just extra
>> locking overhead that can be avoided...
>> 
>> Also, it isn't clear if *freeing* blocks from a group should move it
>> to the end of the same list, or just leave it as-is?  If there are
>> more frees from the list it is likely to be added to a new list soon,
>> and if there are no more frees, then it could stay in the same order.
>> 
>> 
>> Cheers, Andreas
>> 
>> 
>> 
>> 
>> 


Cheers, Andreas






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