DB stress with normal skip list

Summary:
Hash skip list has issues, causing db_stress to fail badly.

For now, switching back to skip_list by default before we figure out root cause.

Test Plan: db_stress is happy(ier)

Reviewers: ljin

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16845

tools/db_crashtest.py

@@ -72,8 +72,8 @@ def main(argv):
             --destroy_db_initially=0
             --reopen=20
             --readpercent=45
-            --prefixpercent=5
-            --writepercent=35
+            --prefixpercent=0
+            --writepercent=40
             --delpercent=5
             --iterpercent=10
             --db=%s
@@ -93,8 +93,8 @@ def main(argv):
             --max_background_compactions=20
             --max_bytes_for_level_base=10485760
             --filter_deletes=%s
-            --memtablerep=prefix_hash
-            --prefix_size=7
+            --memtablerep=skip_list
+            --prefix_size=0
             """ % (ops_per_thread,
                    threads,
                    write_buf_size,

tools/db_crashtest2.py

@@ -86,8 +86,8 @@ def main(argv):
             --destroy_db_initially=0
             --reopen=20
             --readpercent=45
-            --prefixpercent=5
-            --writepercent=35
+            --prefixpercent=0
+            --writepercent=40
             --delpercent=5
             --iterpercent=10
             --db=%s
@@ -107,8 +107,8 @@ def main(argv):
             --max_background_compactions=20
             --max_bytes_for_level_base=10485760
             --filter_deletes=%s
-            --memtablerep=prefix_hash
-            --prefix_size=7
+            --memtablerep=skip_list
+            --prefix_size=0
             %s
             """ % (random.randint(0, 1),
                    threads,

tools/db_stress.cc

@@ -328,7 +328,7 @@ enum RepFactory StringToRepFactory(const char* ctype) {
   return kSkipList;
 }
 static enum RepFactory FLAGS_rep_factory;
-DEFINE_string(memtablerep, "prefix_hash", "");
+DEFINE_string(memtablerep, "skip_list", "");
 
 static bool ValidatePrefixSize(const char* flagname, int32_t value) {
   if (value < 0 || value > 8) {
@@ -1519,12 +1519,6 @@ int main(int argc, char** argv) {
             "not positive!\n");
     exit(1);
   }
-  if (FLAGS_test_batches_snapshots && FLAGS_prefix_size <= 0) {
-    fprintf(stderr,
-            "Error: please specify prefix_size for "
-            "test_batches_snapshots test!\n");
-    exit(1);
-  }
   if ((FLAGS_readpercent + FLAGS_prefixpercent +
        FLAGS_writepercent + FLAGS_delpercent + FLAGS_iterpercent) != 100) {
       fprintf(stderr,