journal: coalesce ftruncate()s in 250ms windows

Prior to this change every journal append causes an ftruncate() for the sake of inotify propagation of the mmap-based writes. With this change the notification is deferred up to ~250ms, coalescing any repeated journal writes during the deferred period into a single ftruncate(). The ftruncate() call isn't free and doing it on every append adds unnecessary overhead and latency in the journald event loop. Introduces journal_file_enable_post_change_timer() which manages a timer on the provided sd-event instance for scheduling coalesced ftruncates. The ftruncate() behavior is unchanged unless journal_file_enable_post_change_timer() is called on the JournalFile. While not a tremendous improvement, profiling systemd-journald event loop latencies using instrumentation as introduced by 34b8751 it was observed that coalescing the ftruncates was low-hanging fruit worth pursuing. Note orders 12 and 13 shifting left into order 11 and order 6 dipping into order 5: Unmodified: log2(us) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 ----------------------------------------------------------- [10685.414572] 0 0 0 0 38 602 61 2 290 60 1643 2554 13 1 4 1 0 0 1 [10690.415114] 0 0 0 0 0 646 54 7 309 44 2073 2148 17 1 3 0 0 0 1 [10695.415509] 0 0 0 0 1 650 73 3 324 37 2071 2270 9 0 0 1 0 1 0 [10700.416297] 0 0 0 0 0 659 50 4 318 38 2111 2152 6 0 1 0 0 1 1 [10705.417136] 0 0 0 0 2 660 48 4 320 38 2129 2146 12 1 1 0 0 1 1 [10710.489114] 0 0 0 0 0 673 38 3 321 37 1925 2339 7 0 0 0 0 1 1 [10715.489613] 0 0 0 0 3 656 64 8 317 48 2365 2007 7 0 0 0 0 0 1 Coalesced: log2(us) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 ----------------------------------------------------------- [ 6169.161360] 0 0 0 1 24 786 54 11 389 24 4192 771 6 4 0 0 1 0 1 [ 6174.161705] 0 0 0 1 18 800 35 6 380 27 3977 893 3 1 0 0 1 0 1 [ 6179.162741] 0 0 0 1 28 768 51 4 391 16 3998 831 5 3 0 0 0 0 2 [ 6184.162856] 0 0 0 0 19 770 60 2 376 26 3795 1004 9 5 1 0 1 0 1 [ 6189.163279] 0 0 0 0 28 761 49 7 372 27 3729 1056 3 2 0 0 1 0 1 [ 6194.164255] 0 0 0 0 25 785 49 7 394 19 3996 908 6 3 2 0 0 0 1 [ 6199.164658] 0 0 0 0 29 797 35 5 389 18 3995 898 3 4 1 1 1 0 1 The remaining high-order delays are a result of the synchronous fsyncs in systemd-journald, beyond the scope of this commit.
2016-01-14 16:36:07 -08:00
parent c57d67f718
commit 7a24f3bf2f
3 changed files with 139 additions and 5 deletions
@@ -39,6 +39,7 @@
 #include "lookup3.h"
 #include "parse-util.h"
 #include "random-util.h"
+#include "sd-event.h"
 #include "string-util.h"
 #include "xattr-util.h"

@@ -149,6 +150,17 @@ JournalFile* journal_file_close(JournalFile *f) {
                journal_file_append_tag(f);
 #endif

+        if (f->post_change_timer) {
+                int enabled;
+
+                if (sd_event_source_get_enabled(f->post_change_timer, &enabled) >= 0)
+                        if (enabled == SD_EVENT_ONESHOT)
+                                journal_file_post_change(f);
+
+                sd_event_source_set_enabled(f->post_change_timer, SD_EVENT_OFF);
+                sd_event_source_unref(f->post_change_timer);
+        }
+
        journal_file_set_offline(f);

        if (f->mmap && f->fd >= 0)
@@ -1400,6 +1412,77 @@ void journal_file_post_change(JournalFile *f) {
                log_error_errno(errno, "Failed to truncate file to its own size: %m");
 }

+static int post_change_thunk(sd_event_source *timer, uint64_t usec, void *userdata) {
+        assert(userdata);
+
+        journal_file_post_change(userdata);
+
+        return 1;
+}
+
+static void schedule_post_change(JournalFile *f) {
+        sd_event_source *timer;
+        int enabled, r;
+        uint64_t now;
+
+        assert(f);
+        assert(f->post_change_timer);
+
+        timer = f->post_change_timer;
+
+        r = sd_event_source_get_enabled(timer, &enabled);
+        if (r < 0) {
+                log_error_errno(-r, "Failed to get ftruncate timer state: %m");
+                return;
+        }
+
+        if (enabled == SD_EVENT_ONESHOT)
+                return;
+
+        r = sd_event_now(sd_event_source_get_event(timer), CLOCK_MONOTONIC, &now);
+        if (r < 0) {
+                log_error_errno(-r, "Failed to get clock's now for scheduling ftruncate: %m");
+                return;
+        }
+
+        r = sd_event_source_set_time(timer, now+f->post_change_timer_period);
+        if (r < 0) {
+                log_error_errno(-r, "Failed to set time for scheduling ftruncate: %m");
+                return;
+        }
+
+        r = sd_event_source_set_enabled(timer, SD_EVENT_ONESHOT);
+        if (r < 0) {
+                log_error_errno(-r, "Failed to enable scheduled ftruncate: %m");
+                return;
+        }
+}
+
+/* Enable coalesced change posting in a timer on the provided sd_event instance */
+int journal_file_enable_post_change_timer(JournalFile *f, sd_event *e, usec_t t) {
+        _cleanup_(sd_event_source_unrefp) sd_event_source *timer = NULL;
+        int r;
+
+        assert(f);
+        assert_return(!f->post_change_timer, -EINVAL);
+        assert(e);
+        assert(t);
+
+        r = sd_event_add_time(e, &timer, CLOCK_MONOTONIC, 0, 0, post_change_thunk, f);
+        if (r < 0)
+                return r;
+
+        r = sd_event_source_set_enabled(timer, SD_EVENT_OFF);
+        if (r < 0)
+                return r;
+
+        f->post_change_timer = timer;
+        timer = NULL;
+        f->post_change_timer_period = t;
+
+        return r;
+}
+
 static int entry_item_cmp(const void *_a, const void *_b) {
        const EntryItem *a = _a, *b = _b;

@@ -1465,7 +1548,10 @@ int journal_file_append_entry(JournalFile *f, const dual_timestamp *ts, const st
        if (mmap_cache_got_sigbus(f->mmap, f->fd))
                r = -EIO;

-        journal_file_post_change(f);
+        if (f->post_change_timer)
+                schedule_post_change(f);
+        else
+                journal_file_post_change(f);

        return r;
 }
@@ -2767,6 +2853,14 @@ int journal_file_open(
                goto fail;
        }

+        if (template && template->post_change_timer) {
+                sd_event *e = sd_event_source_get_event(template->post_change_timer);
+
+                r = journal_file_enable_post_change_timer(f, e, template->post_change_timer_period);
+                if (r < 0)
+                        goto fail;
+        }
+
        *ret = f;
        return 0;

@@ -33,6 +33,7 @@
 #include "journal-def.h"
 #include "macro.h"
 #include "mmap-cache.h"
+#include "sd-event.h"
 #include "sparse-endian.h"

 typedef struct JournalMetrics {
@@ -101,6 +102,9 @@ typedef struct JournalFile {
        JournalMetrics metrics;
        MMapCache *mmap;

+        sd_event_source *post_change_timer;
+        usec_t post_change_timer_period;
+
        OrderedHashmap *chain_cache;

 #if defined(HAVE_XZ) || defined(HAVE_LZ4)
@@ -224,6 +228,7 @@ void journal_file_print_header(JournalFile *f);
 int journal_file_rotate(JournalFile **f, bool compress, bool seal);

 void journal_file_post_change(JournalFile *f);
+int journal_file_enable_post_change_timer(JournalFile *f, sd_event *e, usec_t t);

 void journal_reset_metrics(JournalMetrics *m);
 void journal_default_metrics(JournalMetrics *m, int fd);
@@ -82,6 +82,9 @@

 #define NOTIFY_SNDBUF_SIZE (8*1024*1024)

+/* The period to insert between posting changes for coalescing */
+#define POST_CHANGE_TIMER_INTERVAL_USEC (250*USEC_PER_MSEC)
+
 static int determine_space_for(
                Server *s,
                JournalMetrics *metrics,
@@ -220,6 +223,38 @@ static void server_add_acls(JournalFile *f, uid_t uid) {
 #endif
 }

+static int open_journal(
+                Server *s,
+                bool reliably,
+                const char *fname,
+                int flags,
+                bool seal,
+                JournalMetrics *metrics,
+                JournalFile *template,
+                JournalFile **ret) {
+        int r;
+
+        assert(s);
+        assert(fname);
+        assert(ret);
+
+        if (reliably)
+                r = journal_file_open_reliably(fname, flags, 0640, s->compress, seal, metrics, s->mmap, template, ret);
+        else
+                r = journal_file_open(fname, flags, 0640, s->compress, seal, metrics, s->mmap, template, ret);
+
+        if (r < 0)
+                return r;
+
+        r = journal_file_enable_post_change_timer(*ret, s->event, POST_CHANGE_TIMER_INTERVAL_USEC);
+        if (r < 0) {
+                *ret = journal_file_close(*ret);
+                return r;
+        }
+
+        return r;
+}
+
 static JournalFile* find_journal(Server *s, uid_t uid) {
        _cleanup_free_ char *p = NULL;
        int r;
@@ -258,7 +293,7 @@ static JournalFile* find_journal(Server *s, uid_t uid) {
                journal_file_close(f);
        }

-        r = journal_file_open_reliably(p, O_RDWR|O_CREAT, 0640, s->compress, s->seal, &s->system_metrics, s->mmap, NULL, &f);
+        r = open_journal(s, true, p, O_RDWR|O_CREAT, s->seal, &s->system_metrics, NULL, &f);
        if (r < 0)
                return s->system_journal;

@@ -930,7 +965,7 @@ static int system_journal_open(Server *s, bool flush_requested) {
                (void) mkdir(fn, 0755);

                fn = strjoina(fn, "/system.journal");
-                r = journal_file_open_reliably(fn, O_RDWR|O_CREAT, 0640, s->compress, s->seal, &s->system_metrics, s->mmap, NULL, &s->system_journal);
+                r = open_journal(s, true, fn, O_RDWR|O_CREAT, s->seal, &s->system_metrics, NULL, &s->system_journal);
                if (r >= 0) {
                        server_add_acls(s->system_journal, 0);
                        (void) determine_space_for(s, &s->system_metrics, "/var/log/journal/", "System journal", true, true, NULL, NULL);
@@ -953,7 +988,7 @@ static int system_journal_open(Server *s, bool flush_requested) {
                         * if it already exists, so that we can flush
                         * it into the system journal */

-                        r = journal_file_open(fn, O_RDWR, 0640, s->compress, false, &s->runtime_metrics, s->mmap, NULL, &s->runtime_journal);
+                        r = open_journal(s, false, fn, O_RDWR, false, &s->runtime_metrics, NULL, &s->runtime_journal);
                        if (r < 0) {
                                if (r != -ENOENT)
                                        log_warning_errno(r, "Failed to open runtime journal: %m");
@@ -970,7 +1005,7 @@ static int system_journal_open(Server *s, bool flush_requested) {
                        (void) mkdir("/run/log/journal", 0755);
                        (void) mkdir_parents(fn, 0750);

-                        r = journal_file_open_reliably(fn, O_RDWR|O_CREAT, 0640, s->compress, false, &s->runtime_metrics, s->mmap, NULL, &s->runtime_journal);
+                        r = open_journal(s, true, fn, O_RDWR|O_CREAT, false, &s->runtime_metrics, NULL, &s->runtime_journal);
                        if (r < 0)
                                return log_error_errno(r, "Failed to open runtime journal: %m");
                }