On 7/29/2019 6:20 PM, Josh Steadmon wrote:
trace2 can write files into a target directory. With heavy usage, this
directory can fill up with files, causing difficulty for
trace-processing systems.
When trace2 would write a file to a target directory, first check
whether or not the directory is overloaded. A directory is overloaded if
there is a sentinel file declaring an overload, or if the number of
files exceeds a threshold. If the latter, create a sentinel file to
speed up later overload checks.
Something about this idea bothers me, but I can't quite put my finger
on it. You're filling a directory with thousands of files while
(hopefully simultaneously) having a post-processor/aggregator app
read and delete them. I understand that if the aggregator falls
behind or isn't running, the files will just accumulate and that the
total number of files is the problem. But I have to wonder if
contention on that directory is going to be a bottleneck and/or
a source of problems. That is, you'll have one process reading and
deleting and one or more Git processes scanning/counting/creating.
It seems like there might be opportunity for some kinds of races
here.
It have to wonder if it would be better to do some kind of directory
rotation rather than create a marker file.
Alternatively, I think it would be better to not have the marker
file be inside the directory, but rather have a lock file somewhere
to temporarily disable tracing. Then your stat() call would not need
to effectively search the large directory. Maybe make this
"<dirname>.lock" as a peer to "<dirname>/", for example.
The file count threshold is currently set to 1M files, but this can be
overridden for testing with GIT_TRACE2_TEST_OVERLOAD_FILE_COUNT.
The assumption is that a separate trace-processing system is dealing
with the generated traces; once it processes and removes the sentinel
file, it should be safe to generate new trace files again.
Potential future work:
* Write a message into the sentinel file (should match the requested
trace2 output format).
* Make the overload threshold (and the whole overload feature)
configurable.
I'm wondering if we should just make this setting another
value in `tr2_sysenv_settings[]` rather than a *_TEST_* env var.
That would give you both env and system/global config support,
since I'm assuming you'd eventually want to have this be in
the user's global config with the other trace2 settings.
All of your tests could be expressed in terms of this new setting
and we wouldn't need this new test env var.
Signed-off-by: Josh Steadmon <steadmon@xxxxxxxxxx>
---
t/t0210-trace2-normal.sh | 15 ++++++++
trace2/tr2_dst.c | 81 ++++++++++++++++++++++++++++++++++++++++
2 files changed, 96 insertions(+)
diff --git a/t/t0210-trace2-normal.sh b/t/t0210-trace2-normal.sh
index ce7574edb1..e8a03e9212 100755
--- a/t/t0210-trace2-normal.sh
+++ b/t/t0210-trace2-normal.sh
@@ -186,4 +186,19 @@ test_expect_success 'using global config with include' '
test_cmp expect actual
'
+test_expect_success "don't overload target directory" '
+ GIT_TRACE2_TEST_OVERLOAD_FILE_COUNT=100 &&
+ export GIT_TRACE2_TEST_OVERLOAD_FILE_COUNT &&
+ test_when_finished "rm -r trace_target_dir" &&
+ mkdir trace_target_dir &&
+ test_seq $GIT_TRACE2_TEST_OVERLOAD_FILE_COUNT | sed "s#^#trace_target_dir/#" | sort > expected_filenames.txt &&
+ xargs touch < expected_filenames.txt &&
+ ls trace_target_dir | sed "s#^#trace_target_dir/#" > first_ls_output.txt &&
+ test_cmp expected_filenames.txt first_ls_output.txt &&
+ GIT_TRACE2="$(pwd)/trace_target_dir" test-tool trace2 001return 0 &&
+ echo "trace_target_dir/git-trace2-overload" >> expected_filenames.txt &&
+ ls trace_target_dir | sed "s#^#trace_target_dir/#" > second_ls_output.txt &&
+ test_cmp expected_filenames.txt second_ls_output.txt
+'
+
test_done
diff --git a/trace2/tr2_dst.c b/trace2/tr2_dst.c
index 5dda0ca1cd..3286297918 100644
--- a/trace2/tr2_dst.c
+++ b/trace2/tr2_dst.c
@@ -1,3 +1,5 @@
+#include <dirent.h>
+
#include "cache.h"
#include "trace2/tr2_dst.h"
#include "trace2/tr2_sid.h"
@@ -8,6 +10,18 @@
*/
#define MAX_AUTO_ATTEMPTS 10
+/*
+ * Sentinel file used to detect when we're overloading a directory with too many
+ * trace files.
+ */
+#define OVERLOAD_SENTINEL_NAME "git-trace2-overload"
+
+/*
+ * How many files we can write to a directory before entering overload mode.
+ * This can be overridden with the envvar GIT_TRACE2_TEST_OVERLOAD_FILE_COUNT
+ */
+#define OVERLOAD_FILE_COUNT 1000000
+
static int tr2_dst_want_warning(void)
{
static int tr2env_dst_debug = -1;
@@ -32,6 +46,63 @@ void tr2_dst_trace_disable(struct tr2_dst *dst)
dst->need_close = 0;
}
+/*
+ * Check to make sure we're not overloading the target directory with too many
+ * files. First check for the presence of a sentinel file, then check file
+ * count. If we are overloaded, create the sentinel file if it doesn't already
+ * exist.
+ *
+ * We expect that some trace processing system is gradually collecting files
+ * from the target directory; after it removes the sentinel file we'll start
+ * writing traces again.
+ */
+static int tr2_dst_overloaded(const char *tgt_prefix)
+{
+ int file_count = 0, overload_file_count = 0;
+ char *test_threshold_val;
+ DIR *dirp;
+ struct strbuf path = STRBUF_INIT, sentinel_path = STRBUF_INIT;
+ struct stat statbuf;
+
+ strbuf_addstr(&path, tgt_prefix);
+ if (!is_dir_sep(path.buf[path.len - 1])) {
+ strbuf_addch(&path, '/');
+ }
+
+ /* check sentinel */
+ strbuf_addstr(&sentinel_path, path.buf);
+ strbuf_addstr(&sentinel_path, OVERLOAD_SENTINEL_NAME);
+ if (!stat(sentinel_path.buf, &statbuf)) {
+ strbuf_release(&path);
Also release sentinel_path ?
(And in both of the return statements below.)
+ return 1;
+ }
+
+ /* check if we're overriding the threshold (e.g., for testing) */
+ test_threshold_val = getenv("GIT_TRACE2_TEST_OVERLOAD_FILE_COUNT");
+ if (test_threshold_val)
+ overload_file_count = atoi(test_threshold_val);
+ if (overload_file_count <= 0)
+ overload_file_count = OVERLOAD_FILE_COUNT;
+
+
+ /* check file count */
+ dirp = opendir(path.buf);
+ while (file_count < overload_file_count && dirp && readdir(dirp))
+ file_count++;
+ if (dirp)
+ closedir(dirp);
+
+ if (file_count >= overload_file_count) {
+ creat(sentinel_path.buf, S_IRUSR | S_IWUSR);
+ /* TODO: Write a target-specific message? */
+ strbuf_release(&path);
+ return 1;
+ }
+
+ strbuf_release(&path);
+ return 0;
+}
+
static int tr2_dst_try_auto_path(struct tr2_dst *dst, const char *tgt_prefix)
{
int fd;
@@ -50,6 +121,16 @@ static int tr2_dst_try_auto_path(struct tr2_dst *dst, const char *tgt_prefix)
strbuf_addstr(&path, sid);
base_path_len = path.len;
+ if (tr2_dst_overloaded(tgt_prefix)) {
+ strbuf_release(&path);
+ if (tr2_dst_want_warning())
+ warning("trace2: not opening %s trace file due to too "
+ "many files in target directory %s",
+ tr2_sysenv_display_name(dst->sysenv_var),
+ tgt_prefix);
+ return 0;
+ }
+
for (attempt_count = 0; attempt_count < MAX_AUTO_ATTEMPTS; attempt_count++) {
if (attempt_count > 0) {
strbuf_setlen(&path, base_path_len);
hope this helps,
Jeff
