Re: posix_fadvise noreuse disables file caching

On Wednesday 25 January 2012 17:29:22 John Baldwin wrote:
> On Friday, January 20, 2012 2:12:13 pm John Baldwin wrote:
>> On Thursday, January 19, 2012 11:39:42 am Tijl Coosemans wrote:
>>> I recently noticed that multimedia/vlc generates a lot of disk IO when
>>> playing media files. For instance, when playing a 320kbps mp3 gstat
>>> reports about 1250kBps (=10000kbps). That's quite a lot of overhead.
>>> 
>>> It turns out that vlc sets POSIX_FADV_NOREUSE on the entire file and
>>> reads in chunks of 1028 bytes. FreeBSD implements NOREUSE as if
>>> O_DIRECT was specified during open(2), i.e. it disables all caching.
>>> That means every 1028 byte read turns into a 32KiB read (new default
>>> block size in 9.0) which explains the above numbers.
>>> 
>>> I've copied the relevant vlc code below (modules/access/file.c:Open()).
>>> It's interesting to see that on OSX it sets F_NOCACHE which disables
>>> caching too, but combined with F_RDAHEAD there's still read-ahead
>>> caching.
>>> 
>>> I don't think POSIX intended for NOREUSE to mean O_DIRECT. It should
>>> still cache data (and even do read-ahead if F_RDAHEAD is specified),
>>> and once data is fetched from the cache, it can be marked WONTNEED.
>> 
>> POSIX doesn't specify O_DIRECT, so it's not clear what it asks for.
>> 
>>> Is it possible to implement it this way, or if not to just ignore
>>> the NOREUSE hint for now?
>> 
>> I think it would be good to improve NOREUSE, though I had sort of
>> assumed that applications using NOREUSE would do their own buffering
>> and read full blocks.  We could perhaps reimplement NOREUSE by doing
>> the equivalent of POSIX_FADV_DONTNEED after each read to free buffers
>> and pages after the data is copied out to userland.  I also have an
>> XXX about whether or not NOREUSE should still allow read-ahead as it
>> isn't very clear what the right thing to do there is.  HP-UX (IIRC)
>> has an fadvise() that lets you specify multiple policies, so you
>> could specify both NOREUSE and SEQUENTIAL for a single region to
>> get read-ahead but still release memory once the data is read once.
>
> So I've came up with this untested patch.  It uses
> VOP_ADVISE(FADV_DONTNEED) after read(2) calls to a NOREUSE region, and
> leaves read-ahead caching enabled for NOREUSE.  FADV_DONTNEED doesn't
> do any good really for writes (it only flushes clean buffers), so I've
> left write(2) operations as using IO_DIRECT still.  Does this sound
> reasonable?  I've not yet tested this at all:

The patch drastically improves vlc, but there's still a tiny overhead.
Without NOREUSE the disk is read in chunks of 128KiB (F_RDAHEAD buffer
size). With NOREUSE there's an extra transfer of 32KiB (block size).