Effective implementation of network.¶

For memaslap, both TCP and UDP use non-blocking network IO. All the network events are managed by libevent as memcached. The network module of memaslap is similar to memcached. Libevent can ensure memaslap can handle network very efficiently.

Effective implementation of multi-threads and concurrency¶

Memslap has the similar implementation of multi-threads to memcached. Memslap creates one or more self-governed threads; each thread is bound with one CPU core if the system tests setting CPU core affinity.

In addition, each thread has a libevent to manage the events of the network; each thread has one or more self-governed concurrency; and each concurrency has one or more socket connections. All the concurrent tasks don't communicate with each other even though they are in the same thread.

Memslap can create thousands of socket connections, and each concurrency has tens of socket connections. Each concurrency randomly or sequentially selects one socket connection from its socket connection pool to run, so memaslap can ensure each concurrency handles one socket connection at any given time. Users can specify the number of concurrency and socket connections of each concurrency according to their expected workload.

Effective implementation of generating key and value¶

In order to improve time efficiency and space efficiency, memaslap creates a random characters table with 10M characters. All the suffixes of keys and values are generated from this random characters table.

Memslap uses the offset in the character table and the length of the string to identify a string. It can save much memory. Each key contains two parts, a prefix and a suffix. The prefix is an uint64_t, 8 bytes. In order to verify the data set before, memaslap need to ensure each key is unique, so it uses the prefix to identify a key. The prefix cannot include illegal characters, such as 'r', 'n', '0' and ' '. And memaslap has an algorithm to ensure that.

Memslap doesn't generate all the objects (key-value pairs) at the beginning. It only generates enough objects to fill the task window (default 10K objects) of each concurrency. Each object has the following basic information, key prefix, key suffix offset in the character table, key length, value offset in the character table, and value length.

In the work process, each concurrency sequentially or randomly selects an object from the window to do set operation or get operation. At the same time, each concurrency kicks objects out of its window and adds new object into it.

Simple but useful task scheduling¶

Memslap uses libevent to schedule all concurrent tasks of threads, and each concurrency schedules tasks based on the local task window. Memslap assumes that if each concurrency keeps the same key distribution, value distribution and commands distribution, from outside, memaslap keeps all the distribution as a whole. Each task window includes a lot of objects, each object stores its basic information, such as key, value, expire time, and so on. At any time, all the objects in the window keep the same and fixed key and value distribution. If an object is overwritten, the value of the object will be updated. Memslap verifies the data or expire-time according to the object information stored in the task window.

Libevent selects which concurrency to handle based on a specific network event. Then the concurrency selects which command (get or set) to operate based on the command distribution. If it needs to kick out an old object and add a new object, in order to keep the same key and value distribution, the new object must have the same key length and value length.

If memcached server has two cache layers (memory and SSD), running memaslap with different window sizes can get different cache miss rates. If memaslap adds enough objects into the windows at the beginning, and the cache of memcached cannot store all the objects initialized, then memaslap will get some objects from the second cache layer. It causes the first cache layer to miss. So the user can specify the window size to get the expected miss rate of the first cache layer.

Useful implementation of multi-servers , UDP, TCP, multi-get and binary protocol¶

Because each thread is self-governed, memaslap can assign different threads to handle different memcached servers. This is just one of the ways in which memaslap tests multiple servers. The only limitation is that the number of servers cannot be greater than the number of threads. The other way to test multiple servers is for replication test. Each concurrency has one socket connection to each memcached server. For the implementation, memaslap can set some objects to one memcached server, and get these objects from the other servers.

By default, Memslap does single get. If the user specifies multi-get option, memaslap will collect enough get commands and pack and send the commands together.

Memslap tests both the ASCII protocol and binary protocol, but it runs on the ASCII protocol by default. Memslap by default runs on the TCP protocol, but it also tests UDP. Because UDP is unreliable, dropped packages and out-of-order packages may occur. Memslap creates a memory buffer to handle these problems. Memslap tries to read all the response data of one command from the server and reorders the response data. If some packages get lost, the waiting timeout mechanism can ensure half-baked packages will be discarded and the next command will be sent.

Key size, value size and command distribution.¶

All the distributions are read from the configuration file specified by user with "—cfg_cmd" option. If the user does not specify a configuration file, memaslap will run with the default distribution (key size = 64, value size = 1024, get/set = 9:1). For information on how to edit the configuration file, refer to the "Configuration File" section.

The minimum key size is 16 bytes; the maximum key size is 250 bytes. The precision of proportion is 0.001. The proportion of distribution will be rounded to 3 decimal places.

The minimum value size is 1 bytes; the maximum value size is 1M bytes. The precision of proportion is 0.001. The proportion of distribution will be rounded to 3 decimal places. Currently, memaslap only tests set and get commands. And it testss 100% set and 100% get. For 100% get, it will preset some objects to the server.

Multi-thread and concurrency¶

The high performance of memaslap benefits from the special schedule of thread and concurrency. It's important to specify the proper number of them. The default number of threads is 1; the default number of concurrency is 16. The user can use "—threads" and "--concurrency" to specify these variables.

If the system tests setting CPU affinity and the number of threads specified by the user is greater than 1, memaslap will try to bind each thread to a different CPU core. So if you want to get the best performance memaslap, it is better to specify the number of thread equal to the number of CPU cores. The number of threads specified by the user can also be less or greater than the number of CPU cores. Because of the limitation of implementation, the number of concurrencies could be the multiple of the number of threads.

For example:

--threads=2 --concurrency=128

--threads=8 --concurrency=128

--threads=8 --concurrency=256

--threads=12 --concurrency=144

For example:

--threads=8 --concurrency=128

--threads=16 --concurrency=256

--threads=16 --concurrency=512

--threads=24 --concurrency=288

The memaslap performs very well, when used to test the performance of memcached servers. Most of the time, the bottleneck is the network or the server. If for some reason the user wants to limit the performance of memaslap, there are two ways to do this:

Decrease the number of threads and concurrencies. Use the option "--tps" that memaslap provides to limit the throughput. This option allows the user to get the expected throughput. For example, assume that the maximum throughput is 50 kops/s for a specific configuration, you can specify the throughput equal to or less than the maximum throughput using "--tps" option.

Window size¶

Most of the time, the user does not need to specify the window size. The default window size is 10k. For Schooner Memcached, the user can specify different window sizes to get different cache miss rates based on the test case. Memslap testss cache miss rate between 0% and 100%. If you use this utility to test the performance of Schooner Memcached, you can specify a proper window size to get the expected cache miss rate. The formula for calculating window size is as follows:

Assume that the key size is 128 bytes, and the value size is 2048 bytes, and concurrency=128.

1. Small cache cache_size=1M, 100% cache miss (all data get from SSD). win_size=10k

(1). cache miss rate 0%

win_size=8k

(2). cache miss rate 5%

win_size=11k

(1). cache miss rate 0%

win_size=32k

(2). cache miss

rate 5%

win_size=46k

The formula for calculating window size for cache miss rate 0%:

cache_size / concurrency / (key_size + value_size) * 0.5

The formula for calculating window size for cache miss rate 5%:

cache_size / concurrency / (key_size + value_size) * 0.7

Verification¶

Memslap testss both data verification and expire-time verification. The user can use "--verify=" or "-v" to specify the proportion of data verification. In theory, it testss 100% data verification. The user can use "--exp_verify=" or "-e" to specify the proportion of expire-time verification. In theory, it testss 100% expire-time verification. Specify the "--verbose" options to get more detailed error information.

For example: --exp_verify=0.01 –verify=0.1 , it means that 1% of the objects set with expire-time, 10% of the objects gotten will be verified. If the objects are gotten, memaslap will verify the expire-time and value.

multi-servers and multi-config¶

Memslap testss multi-servers based on self-governed thread. There is a limitation that the number of servers cannot be greater than the number of threads. Memslap assigns one thread to handle one server at least. The user can use the "--servers=" or "-s" option to specify multi-servers.

For example:

--servers=10.1.1.1:11211,10.1.1.2:11212,10.1.1.3:11213 --threads=6 --concurrency=36

The above command means that there are 6 threads, with each thread having 6 concurrencies and that threads 0 and 3 handle server 0 (10.1.1.1); threads 1 and 4 handle server 1 (10.1.1.2); and thread 2 and 5 handle server 2 (10.1.1.3).

All the threads and concurrencies in memaslap are self-governed.

So is memaslap. The user can start up several memaslap instances. The user can run memaslap on different client machines to communicate with the same memcached server at the same. It is recommended that the user start different memaslap on different machines using the same configuration.

Run with execute number mode or time mode¶

The default memaslap runs with time mode. The default run time is 10 minutes. If it times out, memaslap will exit. Do not specify both execute number mode and time mode at the same time; just specify one instead.

For example:

--time=30s (It means the test will run 30 seconds.)

--execute_number=100000 (It means that after running 100000 commands, the test will exit.)

Dump statistic information periodically.¶

The user can use "--stat_freq=" or "-S" to specify the frequency.

For example:

--stat_freq=20s

Memslap will dump the statistics of the commands (get and set) at the frequency of every 20 seconds.

For more information on the format of dumping statistic information, refer to "Format of Output" section.

Multi-get¶

The user can use "--division=" or "-d" to specify multi-get keys count. Memslap by default does single get with TCP. Memslap also testss data verification and expire-time verification for multi-get.

Memslap testss multi-get with both TCP and UDP. Because of the different implementation of the ASCII protocol and binary protocol, there are some differences between the two. For the ASCII protocol, memaslap sends one "multi-get" to the server once. For the binary protocol, memaslap sends several single get commands together as "multi-get" to the server.

UDP and TCP¶

Memslap testss both UDP and TCP. For TCP, memaslap does not reconnect the memcached server if socket connections are lost. If all the socket connections are lost or memcached server crashes, memaslap will exit. If the user specifies the "--reconnect" option when socket connections are lost, it will reconnect them.

User can use "--udp" to enable the UDP feature, but UDP comes with some limitations:

UDP cannot set data more than 1400 bytes.

UDP is not tested by the binary protocol because the binary protocol of memcached does not tests that.

UDP doesn't tests reconnection.

Facebook test¶

Set data with TCP and multi-get with UDP. Specify the following options:

"--facebook --division=50"

If you want to create thousands of TCP connections, specify the

"--conn_sock=" option.

For example: --facebook --division=50 --conn_sock=200

The above command means that memaslap will do facebook test, each concurrency has 200 socket TCP connections and one UDP socket.

Memslap sets objects with the TCP socket, and multi-gets 50 objects once with the UDP socket.

If you specify "--division=50", the key size must be less that 25 bytes because the UDP packet size is 1400 bytes.

Replication test¶

For replication test, the user must specify at least two memcached servers. The user can use "—rep_write=" option to enable feature.

For example:

--servers=10.1.1.1:11211,10.1.1.2:11212 –rep_write=2

The above command means that there are 2 replication memcached servers, memaslap will set objects to both server 0 and server 1, get objects which are set to server 0 before from server 1, and also get objects which are set to server 1 before from server 0. If server 0 crashes, memaslap will only get objects from server 1. If server 0 comes back to life again, memaslap will reconnect server 0. If both server 0 and server 1 crash, memaslap will exit.

Supports thousands of TCP connections¶

Start memaslap with "--conn_sock=" or "-n" to enable this feature. Make sure that your system can tests opening thousands of files and creating thousands of sockets. However, this feature does not tests reconnection if sockets disconnect.

For example:

--threads=8 --concurrency=128 --conn_sock=128

The above command means that memaslap starts up 8 threads, each thread has 16 concurrencies, each concurrency has 128 TCP socket connections, and the total number of TCP socket connections is 128 * 128 = 16384.

Supports binary protocol¶

Start memaslap with "--binary" or "-B" options to enable this feature. It testss all the above features except UDP, because the latest memcached 1.3.3 does not implement binary UDP protocol.

For example:

--binary

Since memcached 1.3.3 doesn't implement binary UDP protocol, memaslap does not tests UDP. In addition, memcached 1.3.3 does not tests multi-get. If you specify "--division=50" option, it just sends 50 get commands together as "multi-get" to the server.

Where¶

servers : "servers"

threads count

concurrency

run time

windows size

set proportion

get proportion

The output of dynamic statistics is something like this:

---------------------------------------------------------------------------------------------------------------------------------
Get Statistics
Type  Time(s)  Ops   TPS(ops/s)  Net(M/s)  Get_miss  Min(us)  Max(us)
Avg(us)  Std_dev    Geo_dist
Period   5   345826  69165     65.3      0         27      2198     203
95.43      177.29
Global  20  1257935  62896     71.8      0         26      3791     224
117.79     192.60
Set Statistics
Type  Time(s)  Ops   TPS(ops/s)  Net(M/s)  Get_miss  Min(us)  Max(us)
Avg(us)  Std_dev    Geo_dist
Period   5    38425   7685      7.3       0         42      628     240
88.05      220.21
Global   20   139780  6989      8.0       0         37      3790    253
117.93     224.83
Total Statistics
Type  Time(s)  Ops   TPS(ops/s)  Net(M/s)  Get_miss  Min(us)  Max(us)
Avg(us)  Std_dev    Geo_dist
Period   5   384252   76850     72.5      0        27      2198     207
94.72      181.18
Global  20  1397720   69886     79.7      0        26      3791     227
117.93     195.60
---------------------------------------------------------------------------------------------------------------------------------

Where¶

Get Statistics

Set Statistics

Total Statistics

Period

Global

Ops

TPS

Net

Get_miss

Min

Max

Avg:

Std_dev

Geo_dist

At the end, memaslap will output something like this:

---------------------------------------------------------------------------------------------------------------------------------
Get Statistics (1257956 events)


  Min:        26


  Max:      3791


  Avg:       224


  Geo:    192.60


  Std:    116.23


                  Log2 Dist:


                    4:        0       10    84490   215345


                    8:   484890   459823    12543      824


                   12:       31


 Set Statistics (139782 events)


    Min:        37


    Max:      3790


    Avg:       253


    Geo:    224.84


    Std:    116.83


    Log2 Dist:


      4:        0        0     4200 16988


      8:    50784    65574 2064      167


      12:        5


  Total Statistics (1397738 events)


      Min:        26


      Max:      3791


      Avg:       227


      Geo:    195.60


      Std:    116.60


      Log2 Dist:


        4:        0       10    88690   232333


        8:   535674   525397    14607      991


        12:       36
cmd_get: 1257969
cmd_set: 139785
get_misses: 0
verify_misses: 0
verify_failed: 0
expired_get: 0
unexpired_unget: 0
written_bytes: 242516030
read_bytes: 1003702556
object_bytes: 152086080
packet_disorder: 0
packet_drop: 0
udp_timeout: 0
Run time: 20.0s Ops: 1397754 TPS: 69817 Net_rate: 59.4M/s
---------------------------------------------------------------------------------------------------------------------------------

Where¶

Get Statistics

Set Statistics

Total Statistics

Min

Max

Avg

Std

Log2 Dist

cmd_get

cmd_set

get_misses

verify_misses

verify_failed

expired_get

unexpired_unget

written_bytes

read_bytes

object_bytes

packet_disorder

packet_drop

udp_timeout

Run time

Ops

TPS

Net_rate