Flush Log every 5 seconds

Summary: This might help with p99 performance, but does not solve the real problem. More discussion on #2947135

Test Plan: make check

Reviewers: dhruba, haobo

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13809

util/env_posix.cc

@@ -1121,7 +1121,7 @@ class PosixEnv : public Env {
     } else {
       int fd = fileno(f);
       SetFD_CLOEXEC(fd, nullptr);
-      result->reset(new PosixLogger(f, &PosixEnv::gettid));
+      result->reset(new PosixLogger(f, &PosixEnv::gettid, this));
       return Status::OK();
     }
   }

util/posix_logger.h

@@ -33,9 +33,13 @@ class PosixLogger : public Logger {
   uint64_t (*gettid_)();  // Return the thread id for the current thread
   std::atomic_size_t log_size_;
   int fd_;
+  const static uint64_t flush_every_seconds_ = 5;
+  uint64_t last_flush_micros_;
+  Env* env_;
  public:
-  PosixLogger(FILE* f, uint64_t (*gettid)()) :
-    file_(f), gettid_(gettid), log_size_(0), fd_(fileno(f)) { }
+  PosixLogger(FILE* f, uint64_t (*gettid)(), Env* env) :
+    file_(f), gettid_(gettid), log_size_(0), fd_(fileno(f)),
+    last_flush_micros_(0), env_(env) { }
   virtual ~PosixLogger() {
     fclose(file_);
   }
@@ -118,7 +122,11 @@ class PosixLogger : public Logger {
       size_t sz = fwrite(base, 1, write_size, file_);
       assert(sz == write_size);
       if (sz > 0) {
-        fflush(file_);
+        if (env_->NowMicros() - last_flush_micros_ >=
+            flush_every_seconds_ * 1000000) {
+          fflush(file_);
+          last_flush_micros_ = env_->NowMicros();
+        }
         log_size_ += write_size;
       }
       if (base != buffer) {