Re: [Xen-devel] [PATCH 3/3] grant-table: defer releasing pages acquired in a grant copy

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.01.15 at 19:19, <david.vrabel@xxxxxxxxxx> wrote:
> Acquiring a page for the source or destination of a grant copy is an
> expensive operation.  A common use case is for two adjacent grant copy
> ops to operate on either the same source or the same destination page.
> 
> Instead of always acquiring and releasing destination and source pages
> for each operation, release the page once it is no longer valid for
> the next op.
> 
> If either the source or destination domains changes both pages are
> released as it is unlikely that either will still be valid.
> 
> XenServer's performance benchmarks show modest improvements in network
> receive throughput (netback uses grant copy in the guest Rx path) and
> no regressions in disk performamnce (using tapdisk3 which grant copies
> as the backend).
> 
>                          Baseline   Deferred Release
> Interhost receive to VM   7.2 Gb/s  ~9 Gbit/s
> Interhost aggregate      24 Gb/s    28 Gb/s
> Intrahost single stream  14 Gb/s    14 Gb/s
> Intrahost aggregate      34 Gb/s    36 Gb/s
> Aggregate disk write    900 MB/s   900 MB/s
> Aggregate disk read     890 MB/s   890 MB/s
> 
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
albeit I wonder ...

> --- a/xen/common/grant_table.c
> +++ b/xen/common/grant_table.c
> @@ -2233,6 +2233,18 @@ static s16 gnttab_copy_claim_buf(
>      return rc;
>  }
>  
> +static bool_t gnttab_copy_buf_valid(const struct gnttab_copy *op,
> +                                    const struct gnttab_copy_ptr *p,
> +                                    const struct gnttab_copy_buf *b,
> +                                    unsigned int gref_flag)
> +{
> +    if ( !b->virt )
> +        return 0;
> +    if ( op->flags & gref_flag )
> +        return b->have_grant && p->u.ref == b->ptr.u.ref;
> +    return p->u.gmfn == b->ptr.u.gmfn;
> +}

... whether you wouldn't better fold the op and gref_flag parameters
into one, as they're used only once and only together.

Jan


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