Re: [PATCH] Pipeline prefetch build-time control

On 15/05/2013 10:20 a.m., Alex Rousskov wrote:
> On 05/11/2013 10:00 PM, Amos Jeffries wrote:
>
>> The attached patch adds a built-time setting SQUID_PIPELINE_MAX to
>> replace the magic numbers previously used for limiting the amount of
>> prefetch Squid does when pipeline_prefeth config directive is enabled.
>> The default remains at 2 unless altered by using -DSQUID_PIPELINE_MAX=n
>> in CXXFLAGS and this adds some documentation about why, and what can be
>> done to improve pipelining in future.
> I do not understand why we need a hard-coded maximum.

I'm not completely sure myself. I think I've checked about half the code
involved and most of it it nicely using linked-list pointer checks. Just
these two input points using magic 2's.

I know that there are protocol use cases where things can happen which
cause the pipeline to needs discarding and that causes all sorts of edge
cases. I'm gettign the idea these 2's were there solely to avoid dealing
with that mess.

PS. When we are certain its safe it will be easy to change
pipeline_prefetch into a numeric directive setting a max limit and some
smart code doing dynamic adjusting. But for now I'm simply not sure
enough about what the rest of the code is doing to go go further than
removing the magic numbers - as we audit and find other code relying on
the magics we can alter them to rely on this macro easily enough.

Amos
Received on Wed May 15 2013 - 10:46:01 MDT