Re: [Qemu-devel] [PATCH 1/4] multifd: Change page count default to 128

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Feb 06, 2019 at 02:23:28PM +0100, Juan Quintela wrote:
> I haven't seend any problem about using 64 or 128.  And it make much
> less contention on the locks.  Just change it.

Isn't there a issue with having a fixed page count given the
very different default page sizes across architectures ?

x86 is 4kb pages, while ppc64 uses 64kb pages IIUC.

This would mean current value of 64 pages, would correspond
to 1/4 MB on x86, and 4 MB on ppc64.  The new value would
be 1/2 MB on x86 and 8 MB on ppc64.

Should we instead be measuring this tunable in units that
are independant of page size ? eg meansure in KB, with a
requirement that the value is a multiple of the page size.
Then set the default to 512 KB ?

> 
> Signed-off-by: Juan Quintela <quintela@redhat.com>
> ---
>  migration/migration.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index ef1d53cde2..f673486679 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -81,7 +81,7 @@
>  /* The delay time (in ms) between two COLO checkpoints */
>  #define DEFAULT_MIGRATE_X_CHECKPOINT_DELAY (200 * 100)
>  #define DEFAULT_MIGRATE_MULTIFD_CHANNELS 2
> -#define DEFAULT_MIGRATE_MULTIFD_PAGE_COUNT 16
> +#define DEFAULT_MIGRATE_MULTIFD_PAGE_COUNT 128
>  
>  /* Background transfer rate for postcopy, 0 means unlimited, note
>   * that page requests can still exceed this limit.
> -- 
> 2.20.1
> 
> 

Regards,
Daniel
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