linux-kernel - Re: [PATCH] fuse: enable larger read buffers for readdir.

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Message-ID: <7d762c95-e4ca-d612-f70f-64789d4624cf@uls.co.za>
Date:   Wed, 26 Jul 2023 17:26:12 +0200
From:   Jaco Kroon <jaco@....co.za>
To:     Bernd Schubert <bernd.schubert@...tmail.fm>,
        Miklos Szeredi <miklos@...redi.hu>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fuse: enable larger read buffers for readdir.

Hi,

On 2023/07/26 15:53, Bernd Schubert wrote:
>
>
> On 7/26/23 12:59, Jaco Kroon wrote:
>> Signed-off-by: Jaco Kroon <jaco@....co.za>
>> ---
>>   fs/fuse/Kconfig   | 16 ++++++++++++++++
>>   fs/fuse/readdir.c | 42 ++++++++++++++++++++++++------------------
>>   2 files changed, 40 insertions(+), 18 deletions(-)
>>
>> diff --git a/fs/fuse/Kconfig b/fs/fuse/Kconfig
>> index 038ed0b9aaa5..0783f9ee5cd3 100644
>> --- a/fs/fuse/Kconfig
>> +++ b/fs/fuse/Kconfig
>> @@ -18,6 +18,22 @@ config FUSE_FS
>>         If you want to develop a userspace FS, or if you want to use
>>         a filesystem based on FUSE, answer Y or M.
>>   +config FUSE_READDIR_ORDER
>> +    int
>> +    range 0 5
>> +    default 5
>> +    help
>> +        readdir performance varies greatly depending on the size of 
>> the read.
>> +        Larger buffers results in larger reads, thus fewer reads and 
>> higher
>> +        performance in return.
>> +
>> +        You may want to reduce this value on seriously constrained 
>> memory
>> +        systems where 128KiB (assuming 4KiB pages) cache pages is 
>> not ideal.
>> +
>> +        This value reprents the order of the number of pages to 
>> allocate (ie,
>> +        the shift value).  A value of 0 is thus 1 page (4KiB) where 
>> 5 is 32
>> +        pages (128KiB).
>> +
>
> I like the idea of a larger readdir size, but shouldn't that be a 
> server/daemon/library decision which size to use, instead of kernel 
> compile time? So should be part of FUSE_INIT negotiation?

Yes sure, but there still needs to be a default.  And one page at a time 
doesn't cut it.

-- snip --

>>   -    page = alloc_page(GFP_KERNEL);
>> +    page = alloc_pages(GFP_KERNEL, READDIR_PAGES_ORDER);
>
> I guess that should become folio alloc(), one way or the other. Now I 
> think order 0 was chosen before to avoid risk of allocation failure. I 
> guess it might work to try a large size and to fall back to 0 when 
> that failed. Or fail back to the slower vmalloc.

If this varies then a bunch of other code will become somewhat more 
complex, especially if one alloc succeeds, and then a follow-up succeeds.

I'm not familiar with the differences between the different mechanisms 
available for allocation.

-- snip --

> Thanks,
My pleasure,
Jaco