Re: [PATCH] libio: Always use _IO_BUFSIZE for stream buffers [BZ #4099]

[Carlos O'Donell <carlos at redhat dot com>]

On 04/01/2016 02:19 PM, Rich Felker wrote:
> On Fri, Mar 18, 2016 at 03:52:58PM -0700, Roland McGrath wrote:
>>> I can do some benchmarking, but I don't expect any compelling results.
>>
>> Whatever the results, they would not IMHO be relevant here.
>>
>> POSIX specifies that st_blksize is the "preferred I/O block size for this
>> object".  It's the kernel's responsibility to give userland good advice
>> through this channel.  If there are common buggy kernels that give bad
>> advice, that is a reason to apply upper and lower limits to the advice from
>> the kernel.  But the expectation should be that the kernel gets fixed to
>> give good advice, and the optimal thing to do with a good kernel is to
>> follow its advice.  
>>
>> Since the recommended use of st_blksize in this way is a standard user
>> feature and not just what stdio's implementation happens to do, there is an
>> argument to be made that the limiting of the value should be done in the
>> *stat functions reported st_blksize values rather than in stdio's use of
>> them.  (I'm ambivalent about this point.)
> 
> Regardless of st_blksize being "the preferred size", it's not suitable
> for stdio, at least not for read purposes, because sparse/random
> access reads are a valid application usage for stdio. Reading an
> unboundedly large "optimal" block size, only to use one byte and throw
> the rest away, is unacceptably pessimistic behavior and is the whole
> point behind bug 4099.
> 
> If you insist on keeping unboundedly large buffers honoring
> st_blksize, one option would be to only use the full buffer for
> writing, and limit it to 4k or 8k for reading. But I think it's best
> to just ignore st_blksize and use a reasonable buffer size all the
> time.

I think Roland has a good point though, if the kernel is going to buffer
for you using filesystem-based knowledge, then why doesn't it just report
an st_blksize that's small, say 8192 bytes, given the implementation?
What purpose does it serve to set st_blksize to 2MB?

-- 
Cheers,
Carlos.