Re: [PATCH v2] btrfs: speedup mount time with readahead chunk tree

[Robbie Ko <robbieko@synology.com>]

David Sterba 於 2020/7/9 下午5:13 寫道:
> On Thu, Jul 09, 2020 at 10:38:42AM +0800, Robbie Ko wrote:
>> David Sterba 於 2020/7/9 上午5:11 寫道:
>>> On Tue, Jul 07, 2020 at 09:25:11PM +0200, David Sterba wrote:
>>>> On Tue, Jul 07, 2020 at 11:59:44AM +0800, robbieko wrote:
>>>> This skips readahead of all nodes above level 1, if you find a nicer way
>>>> to readahead the whole tree I won't object, but for the first
>>>> implementation the level 1 seems ok to me.
>>> Patch below, I tried to create large system chunk by fallocate on a
>>> sparse loop device, but got only 1 node on level 1 so the readahead
>>> cannot show off.
>>>
>>> # btrfs fi df .
>>> Data, single: total=59.83TiB, used=59.83TiB
>>> System, single: total=36.00MiB, used=6.20MiB
>>> Metadata, single: total=1.01GiB, used=91.78MiB
>>> GlobalReserve, single: total=26.80MiB, used=0.00B
>>>
>>> There were 395 leaf nodes that got read ahead, time between the first
>>> and last is 0.83s and the block group tree read took about 40 seconds.
>>> This was in a VM with file-backed images, and the loop device was
>>> constructed from these devices so it's spinning rust.
>>>
>>> I don't have results for non-prefetched mount to compare at the moment.
>>>
>> I think what you're doing is working.
>>
>> But there are many similar problems that need to be improved.
> Agreed, but this started from 'let's readahead chunk tree' and now we
> drifted to fixing or perhaps making better use of readahead in several
> other areas.
>
>> 1. load_free_space_tree
>> We need to read all BTRFS_FREE_SPACE_BITMAP_KEY and
>> BTRFS_FREE_SPACE_EXTENT_KEY until the next FREE_SPACE_INFO_KEY.
>>
>> 2. populate_free_space_tree
>> We need to read all BTRFS_EXTENT_ITEM_KEY and BTRFS_METADATA_ITEM_KEY
>> until the end of the block group
>>
>> 3. btrfs_real_readdir
>> We need as many reads as possible (inode, BTRFS_DIR_INDEX_KEY).
>>
>> 4. btrfs_clone
>> We need as many reads as possible (inode, BTRFS_EXTENT_DATA_KEY).
>>
>> 5. btrfs_verify_dev_extents
>> We need to read all the BTRFS_DEV_EXTENT_KEYs.
> Each case needs to be evaluated separately because the items live in
> different trees and other item types could be scattered among the ones
> we're interested in.
>
> But the list gives an insight in what types of readahead we might need,
> like full key range [from, to], or just all items of one key type.
>
>> 6. caching_kthread (inode-map.c)
>> We need all the BTRFS_INODE_ITEM_KEY of fs_tree to build the inode map
>>
>> For the above cases.
>> It is not possible to write a special readahead code for each case.
>> We have to provide a new readaread framework
>> Enable the caller to determine the scope of readaheads needed.
>> The possible parameters of the readahead are as follows
>> 1. reada_maximum_nr : Read a maximum of several leaves at a time.
>> 2. reada_max_key : READA_FORWARD Early Suspension Condition
>> 3. reada_min_key : READA_BACK Abort condition ahead of time.
> Yeah something like that.
>
>> We need to review all users of readahead to confirm that the The
>> behavior of readahead.
>> For example, in scrub_enumerate_chunks readahead has the effect of Very
>> small,
>> Because most of the time is spent on scrub_chunk,
>> The processing of scrub_chunk for all DEV_EXTENT in a leaf is A long time.
>> If the dev tree has been modified in the meantime, the previously
>> pre-reading leaf may be useless.
> Yes that's another case where doing the readahead is useless.
>
> So, now it's a question if we should start with the easy cases with
> specific readahead and then unify them under a common API, or try to
> figure out the API and then audit all us.
>
> I'd be more in favor of the former as it allows to give us a baseline
> where the readahead would be implemented optimally, the followup API
> cleanup would need to keep the performance.
>
> The latter is IMHO harder just because getting an API right on the first
> try usually does not work.

Okay, I agree with you.
Please help submit your patch to speedup chunk tree read.
Thank you for your help.