On Fri, Apr 15, 2016 at 09:38:42 +0100, Richard W.M. Jones wrote:
In a few places in libvirt we busy-wait for events, for example qemu creating a monitor socket. This is problematic because:
- We need to choose a sufficiently small polling period so that libvirt doesn't add unnecessary delays.
- We need to choose a sufficiently large polling period so that the effect of busy-waiting doesn't affect the system.
The solution to this conflict is to use an exponential backoff.
This patch adds a macro VIR_TIME_WHILE_WITH_BACKOFF to hide the details, and modifies a few places where we currently busy-wait. --- src/fdstream.c | 10 +++++---- src/libvirt_private.syms | 2 ++ src/qemu/qemu_agent.c | 9 ++++---- src/qemu/qemu_monitor.c | 10 +++++---- src/util/virtime.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++ src/util/virtime.h | 34 ++++++++++++++++++++++++++++++ 6 files changed, 107 insertions(+), 12 deletions(-)
diff --git a/src/fdstream.c b/src/fdstream.c index ef118b5..d9646d0 100644 --- a/src/fdstream.c +++ b/src/fdstream.c @@ -42,6 +42,7 @@ #include "virfile.h" #include "configmake.h" #include "virstring.h" +#include "virtime.h"
#define VIR_FROM_THIS VIR_FROM_STREAMS
@@ -520,8 +521,7 @@ int virFDStreamConnectUNIX(virStreamPtr st, bool abstract) { struct sockaddr_un sa; - size_t i = 0; - int timeout = 3; + virTimeBackOffVar timeout; int ret;
int fd = socket(AF_UNIX, SOCK_STREAM, 0); @@ -541,7 +541,9 @@ int virFDStreamConnectUNIX(virStreamPtr st, goto error; }
- do { + if (virTimeBackOffStart(&timeout, 1, 3*1000 /* ms */) < 0) + goto error; + VIR_TIME_WHILE_WITH_BACKOFF(timeout) {
Uff, I don't think we should go this route. Open coding the ugly macro makes the code nicer and easier to read and understand: if (virTimeBackOffStart(&timeout, 1, 3*1000) < 0) goto error; while (virTimeBackOffCondition(&timeout)) { ... } However, I'd suggest changing the name virTimeBackOffCondition to something like virTimeBackOffWait to make it sound better when used in the while loop: while (virTimeBackOffWait(&timeout)) ... The rest of the patch looks fine, though. Jirka