Re: [PATCH][2.6-mm] Readahead issues and AIO read speedup

[Andrew Morton <akpm@osdl.org>]

Badari Pulavarty <pbadari@us.ibm.com> wrote:
>
>  I noticed  the exact same thing while testing on database benchmark 
>  on filesystems (without AIO). I added instrumentation in scsi layer to 
>  record the IO pattern and I found that we are doing lots of (4million) 
>  4K reads, in my benchmark run. I was tracing that and found that all
>  those reads are generated by slow read path, since readahead window 
>  is maximally shrunk. When I forced the readahead code to read 16k 
>  (my database pagesize), in case ra window closed - I see 20% improvement 
>  in my benchmark. I asked "Ramchandra Pai" (linuxram@us.ibm.com)
>  to investigate it further.

But if all the file's pages are already in pagecache (a common case)
this patched kernel will consume extra CPU pointlessly poking away at
pagecache.  Reliably shrinking the window to zero is important for this
reason.

If the database pagesize is 16k then the application should be submitting
16k reads, yes?  If so then these should not be creating 4k requests at the
device layer!  So what we need to do is to ensure that at least those 16k
worth of pages are submitted in a single chunk.  Without blowing CPU if
everything is cached.  Tricky.  I'll take a look at what's going on.


Another relevant constraint here (and there are lots of subtle constraints
in readahead) is that often database files are fragmented all over the
disk, because they were laid out that way (depends on the database and
how it was set up).  In this case, any extra readahead is a disaster
because it incurs extra seeks, needlessly.