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

[Robbie Ko <robbieko@synology.com>]

Holger Hoffstätte 於 2020/7/8 下午10:57 寫道:
> On 2020-07-08 16:04, David Sterba wrote:
>> On Wed, Jul 08, 2020 at 10:19:22AM +0800, Robbie Ko wrote:
>>> David Sterba 於 2020/7/8 上午3:25 寫道:
>>>> On Tue, Jul 07, 2020 at 11:59:44AM +0800, robbieko wrote:
>>>>> From: Robbie Ko <robbieko@synology.com>
>>>>>
>>>>> When mounting, we always need to read the whole chunk tree,
>>>>> when there are too many chunk items, most of the time is
>>>>> spent on btrfs_read_chunk_tree, because we only read one
>>>>> leaf at a time.
>>>>>
>>>>> It is unreasonable to limit the readahead mechanism to a
>>>>> range of 64k, so we have removed that limit.
>>>>>
>>>>> In addition we added reada_maximum_size to customize the
>>>>> size of the pre-reader, The default is 64k to maintain the
>>>>> original behavior.
>>>>>
>>>>> So we fix this by used readahead mechanism, and set readahead
>>>>> max size to ULLONG_MAX which reads all the leaves after the
>>>>> key in the node when reading a level 1 node.
>>>> The readahead of chunk tree is a special case as we know we will need
>>>> the whole tree, in all other cases the search readahead needs is
>>>> supposed to read only one leaf.
>>>
>>> If, in most cases, readahead requires that only one leaf be read, then
>>> reada_ maximum_size should be nodesize instead of 64k, or use
>>> reada_maximum_ nr (default:1) seems better.
>>>
>>>>
>>>> For that reason I don't want to touch the current path readahead logic
>>>> at all and do the chunk tree readahead in one go instead of the
>>>> per-search.
>>>
>>> I don't know why we don't make the change to readahead, because the 
>>> current
>>> readahead is limited to the logical address in 64k is very 
>>> unreasonable,
>>> and there is a good chance that the logical address of the next leaf
>>> node will
>>> not appear in 64k, so the existing readahead is almost useless.
>>
>> I see and it seems that the assumption about layout and chances
>> succesfuly read blocks ahead is not valid. The logic of readahead could
>> be improved but that would need more performance evaluation.
>
> FWIW I gave this a try and see the following numbers, averaged over 
> multiple
> mount/unmount cycles on spinning rust:
>
> without patch : ~2.7s
> with patch    : ~4.5s
>
> ..ahem..
>
I have the following two questions for you.
1. What is the version you are using?
2. Can you please measure the time of btrfs_read_chunk_tree alone?

I think the problem you are having is that btrfs_read_block_groups is
slowing down because it is using the wrong READA flag, which is causing
a lot of useless IO's when reading the block group.

This can be fixed with the following commit.
btrfs: block-group: don't set the wrong READA flag for 
btrfs_read_block_groups()
https://git.kernel.org/pub/scm/linux/kernel 
/git/torvalds/linux.git/commit/?h=v5.8-rc4& 
id=83fe9e12b0558eae519351cff00da1e06bc054d2

> -h