Re: [RFC] ext4: use kmem_cache for short fname allocation in readdir

["David Wang" <00107082@163.com>]

At 2025-05-30 12:35:16, "Theodore Ts'o" <tytso@mit.edu> wrote:
>On Thu, May 29, 2025 at 10:42:56PM +0800, David Wang wrote:
>> When searching files, ext4_readdir would kzalloc() a fname
>> object for each entry. It would be faster if a dedicated
>> kmem_cache is used for fname.
>> 
>> But fnames are of variable length.
>> 
>> This patch suggests using kmem_cache for fname with short
>> length, and resorting to kzalloc when fname needs larger buffer.
>> Assuming long file names are not very common.
>> 
>> Profiling when searching files in kernel code base, with following
>> command:
>> 	# perf record -g -e cpu-clock --freq=max bash -c \
>> 	"for i in {1..100}; do find ./linux -name notfoundatall > /dev/null; done"
>> And using sample counts as indicator of performance improvement.
>
>I would think a better indicator of performance improvement would be
>to measure the system time when running the find commands.  (i.e.,
>either using getrusange with RUSAGE_CHILDREN or wait3 or wait4).

I did use `time` to compare system time when search files with find,
and I did see slight improvement.  
The std deviation is quite high for the whole `find` process though.

>
>We're trading off some extra memory usage and code complexity with
>less CPU time because entries in the kmem_cache might be more TLB
>friendly.  But this is only really going to be applicable if the
>directory is large enough such that the cycles spent in readdir is
>significant compared to the rest of the userspace program, *and* you
>are reading the directory multiple times (e.g., calling find on a
>directory hierarchy many, many times) such that the disk blocks are
>cahed and you don't need to read them from the storage device.
>Otherwise the I/O costs will completely dominate and swamp the
>marginal TLB cache savings.

Yes, the test was run with cache-hot. 
But repeating search files is not  uncommon practice,  `find` would run with cache-hot
except the first round.
 
>
>Given that it's really rare for readdir() to be the bottleneck of many
>workloads, the question is, is it worth it?

That's the question I have been thinking about.
Beside marginal improvement for readdir(), I would argue with the impact on other parts in system
when searching files. Even with cache-code, searching large dir would involving high frequent of
malloc() for a short interval,  This might have transient negative impact  on others which also request malloc(), but with
low frequency.  But I don't have a convincing examples for this, it's all theoretical .


Thanks
David

>
>						- Ted