On Tue, 2011-11-08 at 16:51 +0000, Shriram Rajagopalan wrote:
> On Fri, Nov 4, 2011 at 12:21 PM, Shriram Rajagopalan
> <rshriram@xxxxxxxxx> wrote:
> Why posix_memalign?
>
> The compression code involves a lot of memcpys at 4K
> granularity (dirty pages
> copied from domU's memory to internal cache/page buffers etc).
> I would like to
> keep these memcpys page aligned for purposes of speed. The
> source pages
> (from domU) are already aligned. The destination pages
> allocated by the
> compression code need to be page aligned.
>
> correct me if I am wrong:
> mallocing a huge buffer for this purpose is not optimal.
> malloc aligns allocations
> on 16byte (or 8byte) granularity but if a 4K region straddles
> across two physical
> memory frames, then the memcpy is going to be suboptimal.
> OTOH, memalign
> ensures that we are dealing with just 2 memory frames as
> opposed
> to 3 (possible) frames in malloc.
>
> A simple 8Mb memcpy test shows an average of 500us overhead
> for malloc
> based allocation compared to posix_memalign based allocation.
> While this
> might seem low, the checkpoints are being taken at high
> frequency
> (every 20ms for instance).
>
> It is not okay to use malloc on other platforms. I simply dont
> have access to other
> platforms to test their equivalent versions. Short of using
> something
> like qemu_memalign function.
>
> I am open to suggestions :)
This is due to minios (aka stubdoms) not having posix_memalign, right?
minios (or rather newlib) does appear to have memalign though, which if
true would also work, right? You could potentially also implement
posix_memalign in terms of memalign on minios and avoid the ifdef.
Ian.
>
> shriram
>
>
>
> Ping.
>
>
> On Fri, Nov 4, 2011 at 5:14 AM, Ian Jackson
> <Ian.Jackson@xxxxxxxxxxxxx> wrote:
> rshriram@xxxxxxxxx writes ("[PATCH 0 of 2 V5] libxc:
> checkpoint compression"):
> > This patch series adds checkpoint compression
> functionality, while
> > running under Remus.
>
> ...
> > Changes since last version:
> > 1. use posix_memalign only on linux platforms and
> switch to normal malloc for
> > the rest. stubdom compiles successfully.
>
>
> Looking at this in more detail, I don't understand why
> you're using
> posix_memalign rather than just malloc, anyway. If
> it's necessary to
> use posix_memalign on Linux, why is it OK to use
> malloc on other
> platforms ?
>
> Also this #ifdef is quite ugly.
>
> Ian.
>
>
>
>
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