Re: Unneeded Code Found??

[Randy Appleton]

Nick Piggin wrote:

> > > Randy Appleton wrote:
> > >
> > > > I think I have found some useless code in the Linux kernel
> > > > in the block request functions.
> > > >                                                                                         
> > > >
> > > > I have modified the __make_request function in ll_rw_blk.c.
> > > > Now every request for a block off the hard drive is logged.
> > > >                                                                                         
> > > >
> > > > The function __make_request has code to attempt to merge the current
> > > > block request with some contigious existing request for better
> > > > performance. This merge function keeps a one-entry cache pointing 
> > > > to the
> > > > last block request made.  An attempt is made to merge the current
> > > > request with the last request, and if that is not possible then
> > > > a search of the whole queue is done, looking at merger possibililites.
> > > >                                                                                         
> > > >
> > > > Looking at the data from my logs, I notice that over 50% of all 
> > > > requests
> > > > can be merged.  However, a merge only ever happens between the
> > > > current request and the previous one.  It never happens between the
> > > > current request and any other request that might be in the queue (for
> > > > more than 50,000 requests examined).
> > > >                                                                                         
> > > >
> > > > This is true for several test runs, including "daily usage" and doing
> > > > two kernel compiles at the same time.  I have only tested on a
> > > > single-CPU machine.
> > Does anyone know that this code is actualy useful?  Has anyone ever 
> > seen it actually do a merge of consecutive
> > data accesses for requests that were not issued themselves 
> > consequtively?
> Yes it gets used.
>
> I think its a lot more common with direct io and when you have lots of
> processes.

I'm not arguing, but how do you know this?  I'm trying to convince 
myself that the code is used, and at least on my system
a few days of general use, followed by heavy parallel compiles, doesn't 
use the code even once.

I have not tested direct I/O.  Otherwise it looks unused.

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