ADO.Net SQLCommand.ExecuteReader() slows down or hangs

Question

Environment:

Application (written in C# for .Net 4) has up to 10 threads, each thread runs in its own AppDomain. Each thread uses a ADO.Net DataReader that gets the results from the stored procedure on SQL-Server 2008. Also a thread can use ADO.Net to perform a write operation (Bulk Insert). Everything runs on the local machine.

Problem #1:

Occasionally (approximately each 30th run) execution of a thread slows down drastically. That happens when DataReader gets the stored procedure results - SqlCommand.ExecuteReader(). Usually read operation executes in 10 seconds. When it slows down, it executes in 10-20 minutes. SQLProfiler shows that data is being queried, though very slowly.

Callstack of the slowdown (please note that there are no exceptions):

at SNIReadSync(SNI_Conn* , SNI_Packet** , Int32 )
   at SNINativeMethodWrapper.SNIReadSync(SafeHandle pConn, IntPtr& packet, Int32 timeout)
   at System.Data.SqlClient.TdsParserStateObject.ReadSni(DbAsyncResult asyncResult, TdsParserStateObject stateObj)
   at System.Data.SqlClient.TdsParserStateObject.ReadNetworkPacket()
   at System.Data.SqlClient.TdsParserStateObject.ReadBuffer()
   at System.Data.SqlClient.TdsParserStateObject.ReadByteArray(Byte[] buff, Int32 offset, Int32 len)
   at System.Data.SqlClient.TdsParserStateObject.ReadString(Int32 length)
   at System.Data.SqlClient.TdsParser.ReadSqlStringValue(SqlBuffer value, Byte type, Int32 length, Encoding encoding, Boolean isPlp, TdsParserStateObject stateObj)
   at System.Data.SqlClient.TdsParser.ReadSqlValue(SqlBuffer value, SqlMetaDataPriv md, Int32 length, TdsParserStateObject stateObj)
   at System.Data.SqlClient.SqlDataReader.ReadColumnData()
   at System.Data.SqlClient.SqlDataReader.ReadColumnHeader(Int32 i)
   at System.Data.SqlClient.SqlDataReader.ReadColumn(Int32 i, Boolean setTimeout)
   at System.Data.SqlClient.SqlDataReader.GetValueInternal(Int32 i)
   at System.Data.SqlClient.SqlDataReader.GetValue(Int32 i)
   at System.Data.SqlClient.SqlDataReader.get_Item(String name)
   at ****.Core.TableDataImporter.ImportDataFromExcel(Int32 tableId, ExcelEntityLocation location, Boolean& updateResult) in …

Problem #2:

Instead of a slowing down a thread can hang.

Callstack:

at SNIReadSync(SNI_Conn* , SNI_Packet** , Int32 )
   at SNINativeMethodWrapper.SNIReadSync(SafeHandle pConn, IntPtr& packet, Int32 timeout)
   at System.Data.SqlClient.TdsParserStateObject.ReadSni(DbAsyncResult asyncResult, TdsParserStateObject stateObj)
   at System.Data.SqlClient.TdsParserStateObject.ReadNetworkPacket()
   at System.Data.SqlClient.TdsParserStateObject.ReadBuffer()
   at System.Data.SqlClient.TdsParserStateObject.ReadByte()
   at System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj)
   at System.Data.SqlClient.SqlDataReader.ConsumeMetaData()
   at System.Data.SqlClient.SqlDataReader.get_MetaData()
   at System.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString)
   at System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult result)
   at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method)
   at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior, String method)
   at System.Data.SqlClient.SqlCommand.ExecuteReader()

Callstacks were acquired using debug tools in the background thread. No exceptions do happen, either a slowdown or hanging up.

SNIReadSync is a mechanism that works on the network level and works with transmitting packets across the network. We have reproduced this problem on local machine, removing network problems from the equation.

We are looking for any input and solutions or workarounds for this slowdowns/hang ups. For now we a planning to detect the slowdown an rerun the operation. Thanks in advance.

I'm appending simplified code for the method as requested:

  public void ImportDataFromExcel()
    {            
        try
        {                
            var _сonnectionBuilk = ... ; // singleton connection (at the app level)
            var spName = ... ; // stored procedure name

        var сonnectionToRead = new SqlConnection(connectionStirng);
        сonnectionToRead.Open();

        var sqlCommand = new SqlCommand(spName);
        sqlCommand.CommandType = CommandType.StoredProcedure; 
        sqlCommand.Parameters.Add(param1Name, SqlDbType.Int).Value = ...;
        sqlCommand.Parameters.Add(param2Name, SqlDbType.Int).Value = ...;
        sqlCommand.Parameters.Add(param2Name, SqlDbType.Int).Value = ...;

        sqlCommand.Connection = сonnectionToRead;            
        sqlCommand.CommandTimeout = timeout; // 120 sec

        using (var dataReader = sqlCommand.ExecuteReader())
        {
                dataReader.Read();
            .....
            int pos1 = dataReader.GetOrdinal(columnName1);
            int pos2 = dataReader.GetOrdinal(columnName2);
            int pos3 = dataReader.GetOrdinal(columnName3);
            int pos4 = dataReader.GetOrdinal(columnName4);
                .....                    

            // reading data from sqldatareader
            int val1 = dataReader.GetInt32(pos1);
            int val2 = dataReader.GetInt32(pos2);
            int val3 = dataReader.GetInt32(pos3);
            var val4 = dataReader.GetDateTime(pos4);
            .....

            // append read data into bulkTable
            bulkTable.AddCellValue(val1, val2, val3, val4);  // bulkTable wraps DataTable, and appends DataRow inside. 

            if(bulkTable.DataTable.Rows > MaxRowsCount)
            {
                using (var bulkCopy = new SqlBulkCopy(_сonnectionBuilk))
                {
                    bulkCopy.DestinationTableName = _fullTableName;
                    bulkCopy.WriteToServer(bulkTable.DataTable);
                }

                var sqlCommandTransfer = new SqlCommand(spName);
                sqlCommandTransfer.CommandType = CommandType.StoredProcedure; 
                sqlCommandTransfer.Parameters.Add(param1Name, SqlDbType.Int).Value = ...;
                sqlCommandTransfer.Connection = _сonnectionBuilk;
                ....
                sqlCommandTransfer.ExecuteNonQuery(); // transfering data from temp bulk table into original table
            }
        }
    }
    finally
    {
        bulkTable.Dispose();
        сonnectionToRead.Close();
    }
}

Answer 1

We've been trying to debug a similar issue for months, and finally tracked it down today...

We had a query that was being stashed into the cache (without calliong ToList/ToArray/etc. on it). The query was effectively tied to a connection that had since been cleaned up, and we got what appeared to be 100% CPU blocking from ReadSni (full stack included below).

I suspect the caching code was written before the query was changed over to use Linq (and used to instead return a List<T>, but still cast as IEumberable) so it was introduced when someone made the data access "lazy".

I can't explain why it only happened every few days in production; either the caching isn't being used a lot, or the connection had to be in a certain state for it to fail in this way.

OS Thread Id: 0x20b8 (27)
Child SP IP       Call Site
16edd0fc 6184267e System.Data.SqlClient.TdsParserStateObject.ReadSni(System.Data.Common.DbAsyncResult, System.Data.SqlClient.TdsParserStateObject)
16edd134 61842624 System.Data.SqlClient.TdsParserStateObject.ReadNetworkPacket()
16edd144 618446af System.Data.SqlClient.TdsParserStateObject.ReadBuffer()
16edd150 61c583d0 System.Data.SqlClient.TdsParserStateObject.CleanWire()
16edd15c 61d1beb9 System.Data.SqlClient.TdsParser.Deactivate(Boolean)
16edd174 6184995f System.Data.SqlClient.SqlInternalConnectionTds.InternalDeactivate()
16edd180 61849640 System.Data.SqlClient.SqlInternalConnection.Deactivate()
16edd1b0 61849587 System.Data.ProviderBase.DbConnectionInternal.DeactivateConnection()
16edd1e4 61849405 System.Data.ProviderBase.DbConnectionPool.DeactivateObject(System.Data.ProviderBase.DbConnectionInternal)
16edd224 61849384 System.Data.ProviderBase.DbConnectionPool.PutObject(System.Data.ProviderBase.DbConnectionInternal, System.Object)
16edd26c 6184920c System.Data.ProviderBase.DbConnectionInternal.CloseConnection(System.Data.Common.DbConnection, System.Data.ProviderBase.DbConnectionFactory)
16edd2ac 618490f7 System.Data.SqlClient.SqlInternalConnection.CloseConnection(System.Data.Common.DbConnection, System.Data.ProviderBase.DbConnectionFactory)
16edd2c4 618393bf System.Data.SqlClient.SqlConnection.Close()
16edd304 11238f0a NHibernate.Connection.ConnectionProvider.CloseConnection(System.Data.IDbConnection)
16edd340 11238eae NHibernate.Connection.DriverConnectionProvider.CloseConnection(System.Data.IDbConnection)
16edd34c 11aceb42 NHibernate.AdoNet.ConnectionManager.CloseConnection()
16edd358 11aceb02 NHibernate.AdoNet.ConnectionManager.AggressiveRelease()
16edd364 11acf783 NHibernate.AdoNet.ConnectionManager.AfterTransaction()
16edd370 11acf6d1 NHibernate.Impl.SessionImpl.AfterTransactionCompletion(Boolean, NHibernate.ITransaction)
16edd3ec 11acf5de NHibernate.AdoNet.ConnectionManager.AfterNonTransactionalQuery(Boolean)
16edd3fc 11acf539 NHibernate.Impl.AbstractSessionImpl.AfterOperation(Boolean)
16edd474 130311e4 NHibernate.Impl.SessionImpl.List(NHibernate.IQueryExpression, NHibernate.Engine.QueryParameters, System.Collections.IList)
16ede51c 13031071 NHibernate.Impl.AbstractSessionImpl.List(NHibernate.IQueryExpression, NHibernate.Engine.QueryParameters)
16ede538 13030b68 NHibernate.Impl.ExpressionQueryImpl.List()
16ede568 13030a47 NHibernate.Linq.DefaultQueryProvider.ExecuteQuery(NHibernate.Linq.NhLinqExpression, NHibernate.IQuery, NHibernate.Linq.NhLinqExpression)
16ede59c 11d4c163 NHibernate.Linq.DefaultQueryProvider.Execute(System.Linq.Expressions.Expression)
16ede5b0 11d4c108 NHibernate.Linq.DefaultQueryProvider.Execute[[System.__Canon, mscorlib]](System.Linq.Expressions.Expression)
16ede5c4 11d4c0a6 Remotion.Linq.QueryableBase`1[[System.__Canon, mscorlib]].GetEnumerator()
16ede5d4 61022108 System.Linq.Enumerable+WhereEnumerableIterator`1[[System.__Canon, mscorlib]].MoveNext()*** WARNING: Unable to verify checksum for System.Core.ni.dll
*** ERROR: Module load completed but symbols could not be loaded for System.Core.ni.dll

16ede5e4 610166ea System.Linq.Buffer`1[[System.__Canon, mscorlib]]..ctor(System.Collections.Generic.IEnumerable`1<System.__Canon>)
16ede620 6122e171 System.Linq.OrderedEnumerable`1+<GetEnumerator>d__0[[System.__Canon, mscorlib]].MoveNext()
16ede63c 79b39758 System.Collections.Generic.List`1[[System.__Canon, mscorlib]]..ctor(System.Collections.Generic.IEnumerable`1<System.__Canon>)*** WARNING: Unable to verify checksum for mscorlib.ni.dll
*** ERROR: Module load completed but symbols could not be loaded for mscorlib.ni.dll

16ede66c 61021acf System.Linq.Enumerable.ToList[[System.__Canon, mscorlib]](System.Collections.Generic.IEnumerable`1<System.__Canon>)

Answer 2

Because the code works perfectly for a while we can narrow it down to:

• Database locking / blocking from your processes and some others.
• Database locking / blocking from your processes only.
• Network connectivity.
• State of the data.
• Disk space or some other seemingly unrelated issue on the server.

I'd say it is likely a database lock / block issue. But you really need to determine this for certain. To do this:

• Make sure there is disk space where the databases are writing to - including the database logs and any other logging.
• Make sure no other processes are using the database.
• Preferably use a local database as well to eliminate network issues.
• You are using .Net 4 - so if you are using Tasks then it will be very easy to make them run synchronously with the overloads. Do this and see if the issue is still there.

Doing all of the above should eliminate the issues - and you can start narrowing it down further from there.