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From Yunkai Zhang <yunkai...@gmail.com>
Subject Re: [DISCUSS] Updates to records.config (default) for v3.4.0
Date Fri, 02 Aug 2013 11:07:56 GMT
On Fri, Aug 2, 2013 at 6:15 PM, Leif Hedstrom <leif@ogre.com> wrote:

>
> On Aug 2, 2013, at 12:01 PM, Yongming Zhao <ming.zym@gmail.com> wrote:
>
> > I'd like to enable that reclaimable free list by default building &
> installation, can we?
>
> I'd prefer not for v3.3.5 / 3.4.0. If you still do, I'd absolutely insist
> on it being off by default. We do not have enough data points for the
> usefulness of this feature, and what impact it might have on other systems
> / platforms / services (or e.g. NUMA).
>
> As you know, I still don't buy the reasons why reclaiming memory is
> necessary. It feels like a duct tape solution to an unknown problem (why is
> it allocating more memory than you want it to? Why are you running other
> applications / tools that are competing for memory? How do you avoid for
> the two to not compete for the same memory?).
>


The reason of why reclaming memory is need is that:

In the freelist, there are different size blocks:

1KB, 2KB, 4KB, 8KB, ...

different size block will be placed into different list.

suppose at some time, 2KB block is hot, so ATS will allocate many 2KB
blocks and put them into 2KB-list.

After a while, 4KB block becomes hot, then ATS will allocate many 4KB
blocks and put them into 4KB-list.

... 8KB ...
... 16KB ...
.... .... ....

After ATS running a long time, the number of allocated blocks in each list
will reach the *peek* level, but at some time, *not all* block is hot!

The blocks which not hot, but have been allocated at *peek* level, is the
root cause of memory wast.

That is why reclaiming memory is necessary for ATS, and why
reclaimable-freelist patch can take effect.



>
> All my $.01 of course.
>
> -- Leif
>
>


-- 
Yunkai Zhang
Work at Taobao

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