netdev - how expensive are mallocs?

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Message-ID: <c0a09e5c0911240957x45abfb26k1611e53f435e5179@mail.gmail.com>
Date:	Tue, 24 Nov 2009 09:57:34 -0800
From:	Andrew Grover <andy.grover@...il.com>
To:	netdev@...r.kernel.org
Subject: how expensive are mallocs?

How much effort generally makes sense to avoid mallocs? For example,
rds has a function that does a kmalloc for an array of scatterlist
entries, which is freed at the bottom of the function. We couldn't
allocate off the stack in all cases (too big), but we could allocate
an array of say 8 scatterlists, and use it if that's big enough,
falling back to kmalloc if it's not.

Is this a good idea?

Also, RDS has its own per-cpu page remainder allocator (see
net/rds/page.c) for kernel send buffers. Would cutting this code and
just using kmalloc be recommended? Doesn't SL?B already do per-cpu
pools?

Does this stuff even matter enough to rise above the noise in benchmarks?

Thanks -- Regards -- Andy
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