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Date:   Thu, 29 Aug 2019 09:53:59 -0400
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     Peikan Tsai <peikantsai@gmail.com>, arve@android.com,
        tkjos@android.com, maco@android.com, christian@brauner.io,
        devel@driverdev.osuosl.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] binder: Use kmem_cache for binder_thread
Message-ID: <20190829135359.GB63638@google.com>
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 <20190829064229.GA30423@kroah.com>
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On Thu, Aug 29, 2019 at 08:42:29AM +0200, Greg KH wrote:
> On Thu, Aug 29, 2019 at 01:49:53PM +0800, Peikan Tsai wrote:
[snip] 
> > The allocated size for each binder_thread is 512 bytes by kzalloc.
> > Because the size of binder_thread is fixed and it's only 304 bytes.
> > It will save 208 bytes per binder_thread when use create a kmem_cache
> > for the binder_thread.
> 
> Are you _sure_ it really will save that much memory?  You want to do
> allocations based on a nice alignment for lots of good reasons,
> especially for something that needs quick accesses.

Alignment can be done for slab allocations, kmem_cache_create() takes an
align argument. I am not sure what the default alignment of objects is
though (probably no default alignment). What is an optimal alignment in your
view?

> Did you test your change on a system that relies on binder and find any
> speed improvement or decrease, and any actual memory savings?
> 
> If so, can you post your results?

That's certainly worth it and I thought of asking for the same, but spoke too
soon!

Independent note: In general I find the internal fragmentation with large
kmalloc()s troubling in the kernel :-(. Say you have a 5000 objects of 512
allocation, each 300 bytes. 212 * 5000 is around 1MB. Which is arguably not
neglible on a small memory system, right?

thanks,

 - Joel