DBStress cleanup

Summary:
*) fixed the comment
*) constant 1 was not casted to 64-bit, which (I think) might cause overflow if we shift it too much
*) default prefix size to be 7, like it was before

Test Plan: compiled

Reviewers: ljin

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16827

tools/db_crashtest.py

@@ -94,7 +94,7 @@ def main(argv):
             --max_bytes_for_level_base=10485760
             --filter_deletes=%s
             --memtablerep=prefix_hash
-            --prefix_size=2
+            --prefix_size=7
             """ % (ops_per_thread,
                    threads,
                    write_buf_size,

tools/db_crashtest2.py

@@ -108,7 +108,7 @@ def main(argv):
             --max_bytes_for_level_base=10485760
             --filter_deletes=%s
             --memtablerep=prefix_hash
-            --prefix_size=2
+            --prefix_size=7
             %s
             """ % (random.randint(0, 1),
                    threads,

tools/db_stress.cc

@@ -338,7 +338,7 @@ static bool ValidatePrefixSize(const char* flagname, int32_t value) {
   }
   return true;
 }
-DEFINE_int32(prefix_size, 2, "Control the prefix size for HashSkipListRep");
+DEFINE_int32(prefix_size, 7, "Control the prefix size for HashSkipListRep");
 static const bool FLAGS_prefix_size_dummy =
   google::RegisterFlagValidator(&FLAGS_prefix_size, &ValidatePrefixSize);
 
@@ -1099,7 +1099,8 @@ class StressTest {
         // OPERATION prefix scan
         // keys are 8 bytes long, prefix size is FLAGS_prefix_size. There are
         // (8 - FLAGS_prefix_size) bytes besides the prefix. So there will
-        // be 2 ^ ((8 - FLAGS_prefix_size * 8) combinations.
+        // be 2 ^ ((8 - FLAGS_prefix_size) * 8) possible keys with the same
+        // prefix
         if (!FLAGS_test_batches_snapshots) {
           Slice prefix = Slice(key.data(), FLAGS_prefix_size);
           read_opts.prefix = &prefix;
@@ -1109,7 +1110,8 @@ class StressTest {
             assert(iter->key().starts_with(prefix));
             ++count;
           }
-          assert(count <= (1 << ((8 - FLAGS_prefix_size) * 8)));
+          assert(count <=
+                 (static_cast<int64_t>(1) << ((8 - FLAGS_prefix_size) * 8)));
           if (iter->status().ok()) {
             thread->stats.AddPrefixes(1, count);
           } else {