Re: [Xen-devel] [RFC PATCH 2/3] Dynamically allocate domain->evtchn, also bump EVTCHNS_PER_BUCKET to 512.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 02.01.13 at 15:27, Wei Liu <Wei.Liu2@xxxxxxxxxx> wrote:
> On Wed, 2013-01-02 at 13:38 +0000, David Vrabel wrote:
>> On 31/12/12 18:22, Wei Liu wrote:
>> > From: Wei Liu <liuw@xxxxxxxxx>
>> 
>> The changeset description needs to say why you're increasing
>> EVTCHNS_PER_BUCKET.  I can't tell why.
>> 
> 
> Here is the tedious maths. My thought was that it is not very
> interesting to see this in a change log, so I dropped it. But I will add
> this in my later re-post of this series...
> 
> #define EVTCHNS_PER_BUCKET ???
> #define NR_EVTCHN_BUCKETS (NR_EVENT_CHANNEL_L3/EVTCHNS_PER_BUCKET)
> d->evtchn = xzalloc_array(struct evtchn *, NR_EVTCHN_BUCKETS);
> 
> We need to allow for 3-level evtchn, so use NR_EVENT_CHANNELS_L3 to
> calculate NR_EVTCHN_BUCKETS.
> 
> For 64 bit build, NR_EVENT_CHANNELS_L3 is 256k. The original value of
> EVTCHNS_PER_BUCKET is 128, which means NR_EVTCHN_BUCKETS=2048, thus
> d->evtchn has size of 2048*8 = 16KB = 4 pages. Given that only Dom0 or
> driver domain will need 3-level event channel, this is really overkill
> for most guests.
> 
> If we bump EVTCHNS_PER_BUCKET to 512, d->evtchn becomes 512 * 8 = 4KB =
> 1 page, which I think is more space efficient for most guests.

But not suitable for allocation through xmalloc() (as it'll end up
doing an order-1 page allocation). Either special case it here, or
BUILD_BUG_ON() the effective size being exactly a page, or we
may look into finally making xmalloc() return a page with no
other tracking data when asked for exactly a page's worth of
bytes.

Jan

> Of course, bumping EVTCHNS_PER_BUCKETS will cause overhead in other
> place. Xen allocates a buckets-size of struct evtchn at a time. So the
> overhead is:
> 
> sizeof(struct evtchn) * (512-128) ~= 18 bytes * 384
> 
> So the conclusion is:
> 
> 1 page + sizeof(struct evtchn) * 384  < 4 pages
> 
> IMHO this is a good reason to bump EVTCHNS_PER_BUCKETS to 512.
> 
> 
> Wei.
> 
>> > Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
>> > ---
>> >  xen/common/event_channel.c |    8 ++++++++
>> >  xen/include/xen/sched.h    |    4 ++--
>> >  2 files changed, 10 insertions(+), 2 deletions(-)
>> > 
>> > diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
>> > index 87e422e..9898f8e 100644
>> > --- a/xen/common/event_channel.c
>> > +++ b/xen/common/event_channel.c
>> > @@ -1172,6 +1172,12 @@ void notify_via_xen_event_channel(struct domain 
>> > *ld, 
> int lport)
>> >  
>> >  int evtchn_init(struct domain *d)
>> >  {
>> > +    d->evtchn = (struct evtchn **)
>> > +        xzalloc_array(struct evtchn *, NR_EVTCHN_BUCKETS);
>> 
>> Don't need the cast here?
>> 
>> David
> 
> 
> 
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