On Wed, Dec 06, 2017 at 10:48:45AM +0100, Paolo Bonzini wrote:
> On 06/12/2017 10:26, Yang Zhong wrote:
> > Hello Paolo,
> >
> > The best option is only trim one time after guest kernel bootup or VM
> > bootup, and as for
> > hotplug/unhotplug operations during the VM running, the trim still can do
> > for each batch
> > memory free because trim will not impact VM performance during VM running
> > status.
> >
> > So, the key point is qemu is hard to know when guest ernel bootup is
> > over. If you have some
> > suggestions, please let me know. thanks!
>
> It shouldn't be hard. Does QEMU's RCU thread actually get any
> significant activity after bootup? Hence the suggestion of keeping
> malloc_trim in the RCU thread, but only do it if some time has passed
> since the last time.
>
> Maybe something like this every time the RCU thread runs:
>
> static uint64_t next_trim_time, last_trim_time;
> if (current time < next_trim_time) {
> next_trim_time -= last_trim_time / 2 /* or higher */
> last_trim_time -= last_trim_time / 2 /* same as previous line */
> } else {
> trim_start_time = current time
> malloc_trim(...)
> last_trim_time = current time - trim_start_time
> next_trim_time = current time + last_trim_time
> }
>
> Where the "2" factor should be tuned so that both your and Shannon's
> scenario work fine.
>
Hello Paolo,
Thanks for your help!
I changed the patch per your advice, and new TEMP patch as below:
static void *call_rcu_thread(void *opaque)
{
struct rcu_head *node;
+ int num=1;
rcu_register_thread();
@@ -272,6 +273,21 @@ static void *call_rcu_thread(void *opaque)
node->func(node);
}
qemu_mutex_unlock_iothread();
+
+ static uint64_t next_trim_time, last_trim_time;
+ int delta=100;
+
+ if ( qemu_clock_get_ns(QEMU_CLOCK_HOST) < next_trim_time ) {
+ next_trim_time -= last_trim_time / delta; /* or higher */
+ last_trim_time -= last_trim_time / delta; /* same as previous
line */
+ } else {
+ uint64_t trim_start_time = qemu_clock_get_ns(QEMU_CLOCK_HOST);
+ malloc_trim(4 * 1024 *1024);
+ last_trim_time = qemu_clock_get_ns(QEMU_CLOCK_HOST) -
trim_start_time;
+ next_trim_time = qemu_clock_get_ns(QEMU_CLOCK_HOST) +
last_trim_time;
+ printf("---------memory
trim----------num=%d------last_trim_time=%ld next_trim_time=%ld --\n",
num++,last_trim_time,next_trim_time);
+ }
+
The print log for your reference
---------memory trim----------num=1------last_trim_time=165000
next_trim_time=1512658205270477000 --
---------memory trim----------num=2------last_trim_time=656000
next_trim_time=1512658205278032000 --
---------memory trim----------num=3------last_trim_time=620000
next_trim_time=1512658205298888000 --
---------memory trim----------num=4------last_trim_time=635000
next_trim_time=1512658205339967000 --
---------memory trim----------num=6------last_trim_time=526000
next_trim_time=1512658207659599000 --
---------memory trim----------num=7------last_trim_time=744000
next_trim_time=1512658208121249000 --
---------memory trim----------num=8------last_trim_time=872000
next_trim_time=1512658208132805000 --
---------memory trim----------num=9------last_trim_time=380000
next_trim_time=1512658208376950000 --
---------memory trim----------num=10------last_trim_time=521000
next_trim_time=1512658210648843000 --
Which show trim cost time less than 1ms and call_rcu_thread() do 10 times
batch free, the trim also 10 times.
I also did below changes:
delta=1000, and
next_trim_time = qemu_clock_get_ns(QEMU_CLOCK_HOST) + delta * last_trim_time
The whole VM bootup will trim 3 times.
Regards,
Yang
> Thanks,
>
> Paolo
