Re: Unneeded Code Found??

[Randy Appleton]

Bill Davidsen 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.
> >                                                                                         
> >
> > I wonder if the code (and CPU time) used to search the entire request
> > queue is actually useful.  Would this be a reasonable candidate for code
> > elimination?
>
>
> If you never get a hit, it means either (a) your test load actually 
> doesn't have one, or (b) the code isn't correctly finding them.


It might be buggy code on my part, but it looks pretty solid to me.   
I'd be happy to show anyone interested.
My load ought to find such a merge, if they happen with any freqency at 
all.  Compiling two kernels
at the same time and "general running" are my two current loads.  The 
disk queue gets to over 70
entries, which is rather high for a personal workstation, and I'm 
searching tens of thousands to accesses
in total.

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?


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