ia64/linux-2.6.18-xen.hg

view Documentation/rtc.txt @ 897:329ea0ccb344

balloon: try harder to balloon up under memory pressure.

Currently if the balloon driver is unable to increase the guest's
reservation it assumes the failure was due to reaching its full
allocation, gives up on the ballooning operation and records the limit
it reached as the "hard limit". The driver will not try again until
the target is set again (even to the same value).

However it is possible that ballooning has in fact failed due to
memory pressure in the host and therefore it is desirable to keep
attempting to reach the target in case memory becomes available. The
most likely scenario is that some guests are ballooning down while
others are ballooning up and therefore there is temporary memory
pressure while things stabilise. You would not expect a well behaved
toolstack to ask a domain to balloon to more than its allocation nor
would you expect it to deliberately over-commit memory by setting
balloon targets which exceed the total host memory.

This patch drops the concept of a hard limit and causes the balloon
driver to retry increasing the reservation on a timer in the same
manner as when decreasing the reservation.

Also if we partially succeed in increasing the reservation
(i.e. receive less pages than we asked for) then we may as well keep
those pages rather than returning them to Xen.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
author Keir Fraser <keir.fraser@citrix.com>
date Fri Jun 05 14:01:20 2009 +0100 (2009-06-05)
parents 831230e53067
children
line source
2 Real Time Clock Driver for Linux
3 ================================
5 All PCs (even Alpha machines) have a Real Time Clock built into them.
6 Usually they are built into the chipset of the computer, but some may
7 actually have a Motorola MC146818 (or clone) on the board. This is the
8 clock that keeps the date and time while your computer is turned off.
10 However it can also be used to generate signals from a slow 2Hz to a
11 relatively fast 8192Hz, in increments of powers of two. These signals
12 are reported by interrupt number 8. (Oh! So *that* is what IRQ 8 is
13 for...) It can also function as a 24hr alarm, raising IRQ 8 when the
14 alarm goes off. The alarm can also be programmed to only check any
15 subset of the three programmable values, meaning that it could be set to
16 ring on the 30th second of the 30th minute of every hour, for example.
17 The clock can also be set to generate an interrupt upon every clock
18 update, thus generating a 1Hz signal.
20 The interrupts are reported via /dev/rtc (major 10, minor 135, read only
21 character device) in the form of an unsigned long. The low byte contains
22 the type of interrupt (update-done, alarm-rang, or periodic) that was
23 raised, and the remaining bytes contain the number of interrupts since
24 the last read. Status information is reported through the pseudo-file
25 /proc/driver/rtc if the /proc filesystem was enabled. The driver has
26 built in locking so that only one process is allowed to have the /dev/rtc
27 interface open at a time.
29 A user process can monitor these interrupts by doing a read(2) or a
30 select(2) on /dev/rtc -- either will block/stop the user process until
31 the next interrupt is received. This is useful for things like
32 reasonably high frequency data acquisition where one doesn't want to
33 burn up 100% CPU by polling gettimeofday etc. etc.
35 At high frequencies, or under high loads, the user process should check
36 the number of interrupts received since the last read to determine if
37 there has been any interrupt "pileup" so to speak. Just for reference, a
38 typical 486-33 running a tight read loop on /dev/rtc will start to suffer
39 occasional interrupt pileup (i.e. > 1 IRQ event since last read) for
40 frequencies above 1024Hz. So you really should check the high bytes
41 of the value you read, especially at frequencies above that of the
42 normal timer interrupt, which is 100Hz.
44 Programming and/or enabling interrupt frequencies greater than 64Hz is
45 only allowed by root. This is perhaps a bit conservative, but we don't want
46 an evil user generating lots of IRQs on a slow 386sx-16, where it might have
47 a negative impact on performance. This 64Hz limit can be changed by writing
48 a different value to /proc/sys/dev/rtc/max-user-freq. Note that the
49 interrupt handler is only a few lines of code to minimize any possibility
50 of this effect.
52 Also, if the kernel time is synchronized with an external source, the
53 kernel will write the time back to the CMOS clock every 11 minutes. In
54 the process of doing this, the kernel briefly turns off RTC periodic
55 interrupts, so be aware of this if you are doing serious work. If you
56 don't synchronize the kernel time with an external source (via ntp or
57 whatever) then the kernel will keep its hands off the RTC, allowing you
58 exclusive access to the device for your applications.
60 The alarm and/or interrupt frequency are programmed into the RTC via
61 various ioctl(2) calls as listed in ./include/linux/rtc.h
62 Rather than write 50 pages describing the ioctl() and so on, it is
63 perhaps more useful to include a small test program that demonstrates
64 how to use them, and demonstrates the features of the driver. This is
65 probably a lot more useful to people interested in writing applications
66 that will be using this driver.
68 Paul Gortmaker
70 -------------------- 8< ---------------- 8< -----------------------------
72 /*
73 * Real Time Clock Driver Test/Example Program
74 *
75 * Compile with:
76 * gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest
77 *
78 * Copyright (C) 1996, Paul Gortmaker.
79 *
80 * Released under the GNU General Public License, version 2,
81 * included herein by reference.
82 *
83 */
85 #include <stdio.h>
86 #include <stdlib.h>
87 #include <linux/rtc.h>
88 #include <sys/ioctl.h>
89 #include <sys/time.h>
90 #include <sys/types.h>
91 #include <fcntl.h>
92 #include <unistd.h>
93 #include <errno.h>
95 int main(void) {
97 int i, fd, retval, irqcount = 0;
98 unsigned long tmp, data;
99 struct rtc_time rtc_tm;
101 fd = open ("/dev/rtc", O_RDONLY);
103 if (fd == -1) {
104 perror("/dev/rtc");
105 exit(errno);
106 }
108 fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n");
110 /* Turn on update interrupts (one per second) */
111 retval = ioctl(fd, RTC_UIE_ON, 0);
112 if (retval == -1) {
113 perror("ioctl");
114 exit(errno);
115 }
117 fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading /dev/rtc:");
118 fflush(stderr);
119 for (i=1; i<6; i++) {
120 /* This read will block */
121 retval = read(fd, &data, sizeof(unsigned long));
122 if (retval == -1) {
123 perror("read");
124 exit(errno);
125 }
126 fprintf(stderr, " %d",i);
127 fflush(stderr);
128 irqcount++;
129 }
131 fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:");
132 fflush(stderr);
133 for (i=1; i<6; i++) {
134 struct timeval tv = {5, 0}; /* 5 second timeout on select */
135 fd_set readfds;
137 FD_ZERO(&readfds);
138 FD_SET(fd, &readfds);
139 /* The select will wait until an RTC interrupt happens. */
140 retval = select(fd+1, &readfds, NULL, NULL, &tv);
141 if (retval == -1) {
142 perror("select");
143 exit(errno);
144 }
145 /* This read won't block unlike the select-less case above. */
146 retval = read(fd, &data, sizeof(unsigned long));
147 if (retval == -1) {
148 perror("read");
149 exit(errno);
150 }
151 fprintf(stderr, " %d",i);
152 fflush(stderr);
153 irqcount++;
154 }
156 /* Turn off update interrupts */
157 retval = ioctl(fd, RTC_UIE_OFF, 0);
158 if (retval == -1) {
159 perror("ioctl");
160 exit(errno);
161 }
163 /* Read the RTC time/date */
164 retval = ioctl(fd, RTC_RD_TIME, &rtc_tm);
165 if (retval == -1) {
166 perror("ioctl");
167 exit(errno);
168 }
170 fprintf(stderr, "\n\nCurrent RTC date/time is %d-%d-%d, %02d:%02d:%02d.\n",
171 rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900,
172 rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
174 /* Set the alarm to 5 sec in the future, and check for rollover */
175 rtc_tm.tm_sec += 5;
176 if (rtc_tm.tm_sec >= 60) {
177 rtc_tm.tm_sec %= 60;
178 rtc_tm.tm_min++;
179 }
180 if (rtc_tm.tm_min == 60) {
181 rtc_tm.tm_min = 0;
182 rtc_tm.tm_hour++;
183 }
184 if (rtc_tm.tm_hour == 24)
185 rtc_tm.tm_hour = 0;
187 retval = ioctl(fd, RTC_ALM_SET, &rtc_tm);
188 if (retval == -1) {
189 perror("ioctl");
190 exit(errno);
191 }
193 /* Read the current alarm settings */
194 retval = ioctl(fd, RTC_ALM_READ, &rtc_tm);
195 if (retval == -1) {
196 perror("ioctl");
197 exit(errno);
198 }
200 fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n",
201 rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec);
203 /* Enable alarm interrupts */
204 retval = ioctl(fd, RTC_AIE_ON, 0);
205 if (retval == -1) {
206 perror("ioctl");
207 exit(errno);
208 }
210 fprintf(stderr, "Waiting 5 seconds for alarm...");
211 fflush(stderr);
212 /* This blocks until the alarm ring causes an interrupt */
213 retval = read(fd, &data, sizeof(unsigned long));
214 if (retval == -1) {
215 perror("read");
216 exit(errno);
217 }
218 irqcount++;
219 fprintf(stderr, " okay. Alarm rang.\n");
221 /* Disable alarm interrupts */
222 retval = ioctl(fd, RTC_AIE_OFF, 0);
223 if (retval == -1) {
224 perror("ioctl");
225 exit(errno);
226 }
228 /* Read periodic IRQ rate */
229 retval = ioctl(fd, RTC_IRQP_READ, &tmp);
230 if (retval == -1) {
231 perror("ioctl");
232 exit(errno);
233 }
234 fprintf(stderr, "\nPeriodic IRQ rate was %ldHz.\n", tmp);
236 fprintf(stderr, "Counting 20 interrupts at:");
237 fflush(stderr);
239 /* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */
240 for (tmp=2; tmp<=64; tmp*=2) {
242 retval = ioctl(fd, RTC_IRQP_SET, tmp);
243 if (retval == -1) {
244 perror("ioctl");
245 exit(errno);
246 }
248 fprintf(stderr, "\n%ldHz:\t", tmp);
249 fflush(stderr);
251 /* Enable periodic interrupts */
252 retval = ioctl(fd, RTC_PIE_ON, 0);
253 if (retval == -1) {
254 perror("ioctl");
255 exit(errno);
256 }
258 for (i=1; i<21; i++) {
259 /* This blocks */
260 retval = read(fd, &data, sizeof(unsigned long));
261 if (retval == -1) {
262 perror("read");
263 exit(errno);
264 }
265 fprintf(stderr, " %d",i);
266 fflush(stderr);
267 irqcount++;
268 }
270 /* Disable periodic interrupts */
271 retval = ioctl(fd, RTC_PIE_OFF, 0);
272 if (retval == -1) {
273 perror("ioctl");
274 exit(errno);
275 }
276 }
278 fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n");
279 fprintf(stderr, "\nTyping \"cat /proc/interrupts\" will show %d more events on IRQ 8.\n\n",
280 irqcount);
282 close(fd);
283 return 0;
285 } /* end main */