iolog: don't copy log samples for compression

For larger settings of log_compression, the allocation and copying of
the log will adversely affect performance and logged latencies. Instead
just reference the log in the compression code, while resetting the
current log to a smaller size. We pay a slightly higher cost in memory
until compression has completd, but it saves us from allocating and
copying tens (or hundreds) of megabytes inline.

Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/iolog.c b/iolog.c
index 27d00ff93c8da91a2e233a93b4209fe895b3d76f..e674171e5195f71b74b6c03514a33d597ad4312e 100644 (file)
--- a/iolog.c
+++ b/iolog.c
@@ -1154,15 +1154,15 @@ void iolog_compress_exit(struct thread_data *td)
 }
 
 /*
- * Queue work item to compress the existing log entries. We copy the
- * samples, and reset the log sample count to 0 (so the logging will
- * continue to use the memory associated with the log). If called with
- * wait == 1, will not return until the log compression has completed.
+ * Queue work item to compress the existing log entries. We reset the
+ * current log to a small size, and reference the existing log in the
+ * data that we queue for compression. Once compression has been done,
+ * this old log is freed. If called with wait == 1, will not return until
+ * the log compression has completed.
  */
 int iolog_flush(struct io_log *log, int wait)
 {
        struct iolog_flush_data *data;
-       size_t sample_size;
 
        data = malloc(sizeof(*data));
        if (!data)
@@ -1170,16 +1170,12 @@ int iolog_flush(struct io_log *log, int wait)
 
        data->log = log;
 
-       sample_size = log->nr_samples * log_entry_sz(log);
-       data->samples = malloc(sample_size);
-       if (!data->samples) {
-               free(data);
-               return 1;
-       }
-
-       memcpy(data->samples, log->log, sample_size);
+       data->samples = log->log;
        data->nr_samples = log->nr_samples;
+
        log->nr_samples = 0;
+       log->max_samples = 128;
+       log->log = malloc(log->max_samples * log_entry_sz(log));
 
        data->wait = wait;
        if (data->wait) {