Re: remove __read_mostly?

[Paul Jackson <pj@sgi.com>]

Ravikiran wrote:
> The idea behind __read_mostly is to separate variables like cpu maps,
> bootcpuinfo etc which are written to very very rarely -- during 
> initialization/hot-plugging, but read quite often something like ~100 % read
> ratio.  

So these variables are __read_hot_write_cold?

In other words, the name __read_mostly is a little misleading, in my
book.  That name only suggests read much more than written.  In your
words:

    something like 99:1 read

It doesn't state that the variable is so "read hot", it is worth keeping
off "write hot" cache lines.

Let's say for example we have a variable is accessed only once per
hour, and that this access is always a read except once a week (once
every 168 hours) when it is a write.

I would not mark that variable __read_mostly, even though it passed
your 99:1 test.  That variable is read_cold_write_evencolder.  It's an
ideal candidate for the canon fodder that we use to fill up the rest
of a cache line that has a hot variable.

If Andrew's suggestion to remove __read_mostly doesn't fly, then I'd
vote for a name change:

  __read_mostly ==> __read_hot_write_cold

I think we want to identify the hottest memory words, keeping them
on separate cache lines, except that __read_hot_write_cold words can
share cache lines with each other.

A given cache line would have at most one hot write word, or one or
more read hot, write cold words.

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@sgi.com> 1.925.600.0401
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