Re: [Xen-devel] Re: [PATCH 0 of 2 V5] libxc: checkpoint compression

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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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