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,
As for your patch, i have commented on another mail.
Please help check below TEMP patch.
+++ b/util/rcu.c
@@ -32,7 +32,7 @@
#include "qemu/atomic.h"
#include "qemu/thread.h"
#include "qemu/main-loop.h"
-
+#if defined(CONFIG_MALLOC_TRIM)
+#include <malloc.h>
+#endif
/*
* Global grace period counter. Bit 0 is always one in rcu_gp_ctr.
* Bits 1 and above are defined in synchronize_rcu.
@@ -246,6 +246,7 @@ static void *call_rcu_thread(void *opaque)
qemu_event_reset(&rcu_call_ready_event);
n = atomic_read(&rcu_call_count);
if (n == 0) {
+ #if defined(CONFIG_MALLOC_TRIM)
+ malloc_trim(4 * 1024 * 1024);
+ #endif
qemu_event_wait(&rcu_call_ready_event);
}
}
If there is no rcu_call(), the n=0 and call_rcu_thread() will trim memory
and then enter sleep to wait for rcu_call() to wakeup this thread.
Once the VM bootup, if there is not any activity like hotplug, rcu thread
is always in sleep status.
As for the VM bootup, if the n!=0, the rcu thread will not call trim.
If use this method, the trim times will decrease to around 1/2 of previous.
Regards,
Yang
> Thanks,
>
> Paolo
