Re: [PATCH v2 0/6] iomap: some minor non-critical fixes and improvements when block size < folio size

[Zhang Yi <yi.zhang@huaweicloud.com>]

On 2024/8/14 13:16, Dave Chinner wrote:
> On Wed, Aug 14, 2024 at 11:57:03AM +0800, Zhang Yi wrote:
>> On 2024/8/14 10:47, Dave Chinner wrote:
>>> On Wed, Aug 14, 2024 at 10:14:01AM +0800, Zhang Yi wrote:
>>>> On 2024/8/14 9:49, Dave Chinner wrote:
>>>>> important to know if the changes made actually provided the benefit
>>>>> we expected them to make....
>>>>>
>>>>> i.e. this is the sort of table of results I'd like to see provided:
>>>>>
>>>>> platform	base		v1		v2
>>>>> x86		524708.0	569218.0	????
>>>>> arm64		801965.0	871605.0	????
>>>>>
>>>>
>>>>  platform	base		v1		v2
>>>>  x86		524708.0	571315.0 	569218.0
>>>>  arm64	801965.0	876077.0	871605.0
>>>
>>> So avoiding the lock cycle in iomap_write_begin() (in patch 5) in
>>> this partial block write workload made no difference to performance
>>> at all, and removing a lock cycle in iomap_write_end provided all
>>> that gain?
>>
>> Yes.
>>
>>>
>>> Is this an overwrite workload or a file extending workload? The
>>> result implies that iomap_block_needs_zeroing() is returning false,
>>> hence it's an overwrite workload and it's reading partial blocks
>>> from disk. i.e. it is doing synchronous RMW cycles from the ramdisk
>>> and so still calling the uptodate bitmap update function rather than
>>> hitting the zeroing case and skipping it.
>>>
>>> Hence I'm just trying to understand what the test is doing because
>>> that tells me what the result should be...
>>>
>>
>> I forgot to mentioned that I test this on xfs with 1K block size, this
>> is a simple case of block size < folio size that I can direct use
>> UnixBench.
> 
> OK. So it's an even more highly contrived microbenchmark than I
> thought. :/
> 
> What is the impact on a 4kB block size filesystem running that same
> 1kB write test? That's going to be a far more common thing to occur
> in production machines for such small IO, 

Yeah, I agree with you, the original test case I want to test is
buffered overwrite with bs=4K to the 4KB filesystem which has existing
larger size folios (> 4KB), this is one kind of common case of
block size < folio size after large folio is enabled. But I don't find
a benchmark tool can do this test easily, so I use the above tests
parameters to simulate this case.

> let's make sure that we
> haven't regressed that case in optimising for this one.

Sure, I will test this case either.

> 
>> This test first do buffered append write with bs=1K,count=2000 in the
>> first round, and then do overwrite from the start position with the same
>> parameters repetitively in 30 seconds. All the write operations are
>> block size aligned, so iomap_write_begin() just continue after
>> iomap_adjust_read_range(), don't call iomap_set_range_uptodate() to set
>> range uptodate originally, hence there is no difference whether with or
>> without patch 5 in this test case.
> 
> Ok, so you really need to come up with an equivalent test that
> exercises the paths that patch 5 modifies, because right now we have
> no real idea of what the impact of that change will be...
> 

Sure.

Thanks,
Yi.