Performance-related: How does SQL Server process concurrent queries from multiple connections from a single .NET desktop application? - sql-server

Question

Single-threaded version description:
Program gathers a list of questions.
For each question, get model answers, and run each one through a scoring module.
Scoring module makes a number of (read-only) database queries.
Serial processing, single database connection.
I decided to multi-thread the above described program by splitting the question list into chunks and creating a thread for each one.
Each thread opens it's own database connection and works on it's own list of questions (about 95 questions on each of 6 threads). The application waits for all threads to finish, then aggregates the results for display.
To my surprise, the multi-threaded version ran in approximately the same time, taking about 16 seconds instead of 17.
Questions:
Why am I not seeing the kind of gain in performance I would expect from executing queries concurrently on separate threads with separate connections? Machine has 8 processors.
Will SQL Server process queries concurrently when they are coming from a single application, or might it (or .net itself) be serializing them?
Might there be something misconfigured, that would make it go faster, or might I just be pushing SQL Server to its computational limits?
Current configuration:
Microsoft SQL Server Developer Edition 9.0.1406 RTM
OS: Windows Server 2003 Standard
Processors: 8
RAM: 4GB

Answer 1

This is just a shot in the dark, but I bet you are not seeing the performance gain because they serialize themselves in the database due to locking of shared resources (records). Now for the small print.
I assume your C# code is actually correct and you actually do start separate threads and issue each query in parallel. No offense, but I've seen many making that claim and the code being actually serial in the client, for various reasons. You should validate this by monitoring the server (via Profiler, or use the sys.dm_exec_requests and sys.dm_exec_sessions).
Also I assume that your queries are of similar weight. i.e., you do not have one thread that lasts 15 seconds and 5 that 100 ms.
The symptoms you describe, in lack of more details, would point that you have a write operation at the beginning of each thread that takes an X lock on some resource. First thread starts and locks the resource, other 5 wait. 1st thread is done, releases the resource then the next one grabs it, other 4 wait. So last thread has to wait for the execution of all other 5. This would be extremely easy to troubleshoot by looking at sys.dm_exec_requests and monitor what blocks the requests.
BTW you should consider using Asynchronous Processing=true and rely on the async methods like BeginExecuteReader to launch your commands in execution in parallel w/o the overhead of client side threads.

Answer 2

You can simply check the task manager when the process is running. If it's showing 100% CPU usage then its CPU bound. Otherwise its IO Bound.
For hyperthreading 50% CPU usage is roughly equal to 100% usage!
Wow I didn't realize how old the thread was. I guess its always good to leave the response for others looking.

Answer 3

How large is your database?
How fast are your HDDs / Raid / Other storage
Perhaps your DB is I/O bound?

Answer 4

My first inclination is that you're trying to solve an IO problem with threads, which almost never works. IO is IO, and more threads doesn't increase the pipe. You'd be better off downloading all questions and their answers in one batch and processing the batch locally with multiple threads.
Having said that, you're probably experiencing some db locking that is causing slowness. Since you're talking about read-only queries, try using the with (nolock) hint on your queries to see if that helps.
Regarding SQL server processing, it is my understanding that SQL Server will try to process as many connections concurrently as possible (one statement at a time per connection), up to the max connections allowed by configuration. The kind if issue you're seeing is almost never a thread issue and almost always a locking or IO problem.

Answer 5

is it possible that the the threads share a connection? did you verify that multiple SPIDs are created when this runs (sp_who)?

Answer 6

I ran a join query across sys.dm_os_workers, sys.dm_os_tasks, and sys.dm_exec_requests on task_address, and here are the results (some uninteresting/zero-valued fields excluded, others prefixed with ex or os to resolve ambiguities):
-COL_NAME- -Thread_1- -Thread_2- -Thread_3- -Thread_4-
task_state SUSPENDED SUSPENDED SUSPENDED SUSPENDED
context_switches_count 2 2 2 2
worker_address 0x3F87A0E8 0x5993E0E8 0x496C00E8 0x366FA0E8
is_in_polling_io_completion_routine 0 0 0 0
pending_io_count 0 0 0 0
pending_io_byte_count 0 0 0 0
pending_io_byte_average 0 0 0 0
wait_started_ms_ticks 1926478171 1926478187 1926478171 1926478187
wait_resumed_ms_ticks 1926478171 1926478187 1926478171 1926478187
task_bound_ms_ticks 1926478171 1926478171 1926478156 1926478171
worker_created_ms_ticks 1926137937 1923739218 1921736640 1926137890
locale 1033 1033 1033 1033
affinity 1 4 8 32
state SUSPENDED SUSPENDED SUSPENDED SUSPENDED
start_quantum 3074730327955210 3074730349757920 3074730321989030 3074730355017750
end_quantum 3074730334339210 3074730356141920 3074730328373030 3074730361401750
quantum_used 6725 11177 11336 6284
max_quantum 4 15 5 20
boost_count 999 999 999 999
tasks_processed_count 765 1939 1424 314
os.task_address 0x006E8A78 0x00AF12E8 0x00B84C58 0x00D2CB68
memory_object_address 0x3F87A040 0x5993E040 0x496C0040 0x366FA040
thread_address 0x7FF08E38 0x7FF8CE38 0x7FF0FE38 0x7FF92E38
signal_worker_address 0x4D7DC0E8 0x571360E8 0x2F8560E8 0x4A9B40E8
scheduler_address 0x006EC040 0x00AF4040 0x00B88040 0x00E40040
os.request_id 0 0 0 0
start_time 2009-05-26 19:39 39:43.2 39:43.2 39:43.2
ex.status suspended suspended suspended suspended
command SELECT SELECT SELECT SELECT
sql_handle 0x020000009355F1004BDC90A51664F9174D245A966E276C61 0x020000009355F1004D8095D234D39F77117E1BBBF8108B26 0x020000009355F100FC902C84A97133874FBE4CA6614C80E5 0x020000009355F100FC902C84A97133874FBE4CA6614C80E5
statement_start_offset 94 94 94 94
statement_end_offset -1 -1 -1 -1
plan_handle 0x060007009355F100B821C414000000000000000000000000 0x060007009355F100B8811331000000000000000000000000 0x060007009355F100B801B259000000000000000000000000 0x060007009355F100B801B259000000000000000000000000
database_id 7 7 7 7
user_id 1 1 1 1
connection_id BABF5455-409B-4F4C-9BA5-B53B35B11062 A2BBCACF-D227-466A-AB08-6EBB56F34FF2 D330EDFE-D49B-4148-B7C5-8D26FE276D30 649F0EC5-CB97-4B37-8D4E-85761847B403
blocking_session_id 0 0 0 0
wait_type CXPACKET CXPACKET CXPACKET CXPACKET
wait_time 46 31 46 31
ex.last_wait_type CXPACKET CXPACKET CXPACKET CXPACKET
wait_resource
open_transaction_count 0 0 0 0
open_resultset_count 1 1 1 1
transaction_id 3052202 3052211 3052196 3052216
context_info 0x 0x 0x 0x
percent_complete 0 0 0 0
estimated_completion_time 0 0 0 0
cpu_time 0 0 0 0
total_elapsed_time 54 41 65 39
reads 0 0 0 0
writes 0 0 0 0
logical_reads 78745 123090 78672 111966
text_size 2147483647 2147483647 2147483647 2147483647
arithabort 0 0 0 0
transaction_isolation_level 2 2 2 2
lock_timeout -1 -1 -1 -1
deadlock_priority 0 0 0 0
row_count 6 0 1 1
prev_error 0 0 0 0
nest_level 2 2 2 2
granted_query_memory 512 512 512 512
The query plan predictor for all queries shows a couple nodes, 0% for select, and 100% for a clustered index seek.
Edit: The fields and values I left out where (same for all 4 threads, except for context_switch_count): exec_context_id(0), host_address(0x00000000), status(0), is_preemptive(0), is_fiber(0), is_sick(0), is_in_cc_exception(0), is_fatal_exception(0), is_inside_catch(0), context_switch_count(3-89078), exception_num(0), exception_Severity(0), exception_address(0x00000000), return_code(0), fiber_address(NULL), language(us_english), date_format(mdy), date_first(7), quoted_identifier(1), ansi_defaults(0), ansi_warnings(1), ansi_padding(1), ansi_nulls(1), concat_null_yields_null(1), executing_managed_code(0)