[squid-dev] [RFC][PATCH] Bug4438 second attempt: give MemBlob its own pools

[Kinkie]

On Tue, Mar 15, 2016 at 9:43 PM, Alex Rousskov
<rousskov at measurement-factory.com> wrote:
> On 03/14/2016 02:11 PM, Kinkie wrote:
>
>>   this second attempt at bug4438 tries a different approach: by giving
>> MemBlob its own pools and having a hard initialization dependency in
>> MemBlob's allocating function, instead of relying on memAllocString.
>
> Forgive me if I missed how this decision was made earlier, but I do not
> understand why we want to duplicate the existing memAllocString()-like
> functionality and split a single set of raw memory pools shared by all
> callers into two isolated sets?

It is an attempt to sidestep initialization issues by having pools in
the same translation unit as their user.
The other effects are IMO generally beneficial but are not the main goal.

> In other words, how did we get from "SIGSEGV in memFreeString" (i.e.,
> bug 4438) to this? If your working (or perhaps already proven!) theory
> is that memAllocString() does something wrong, why not fix
> memAllocString() instead of duplicating it? I must be missing something
> obvious here...

Unfortunately we can't prove what is doing what wrong as we (or at
least I) can't reproduce the bug.

>> Known side-effects:
>> - allows to tune memblob sizes according to actual use patterns, which
>> can be collected via SBuf/MemBlob detailed stats. The currently-set
>> sizes follow some light testing I've done.
>
> Why is that a side effect? Could we not "tune memblob sizes according to
> actual use patterns" before this change?

We do not know the useage patterns of all memAllocString callers; we
do know those of MemBlob via SBufDetailedStats. As an alternative
approach for this kind of optimization, we could instrument
memAllocString to gather that data as well.

> Or do you meant that we can tune two separate sets of pools now? If yes,
> then you should also list "two separate sets of pools instead of one
> shared by all callers" as a side effect -- "more isolated sets" is the
> "cost" associated with this ability to tune more sets...

Nod.

>> - as no MemBlob can be initialized before MemPools, there is no need
>> to baloon statically-initialized MemBlobs to
>> SmallestStringBeforeMemIsInitialized.
>
> If your change guarantees that no MemBlob can be initialized before
> MemPools, why cannot a similar change be done to the original code so
> that no memAllocString()/etc. caller can get its result before MemPools
> are initialized? AFAICT, you have solved the problem that you were
> trying to solve, but I do not understand why you had to solve it in a
> new location (leaving the old problem unsolved?).

I do not yet know if I have solved the problem, unfortunately.
If this attempt is successful, it can be a blueprint for modernizing
memAllocString, or splitting other callers altogether. The file name
(old_api) is to me an indicator that these API are not really
encouraged, and on the way to deprecation if not deprecated already.

Thanks

-- 
    Francesco