Bad Django / uwsgi performance

Question

I am running a django app with nginx & uwsgi. Here\'s how i run uwsgi:

sudo uwsgi -b 25000 --chdir=/www/python/apps/pyapp --module=wsgi:application --env DJA         


        

Answer 1

EDIT 1 Seen the comment that you have 1 virtual core, adding commentary through on all relavant points

EDIT 2 More information from Maverick, so I'm eliminating ideas ruled out and developing the confirmed issues.

EDIT 3 Filled out more details about uwsgi request queue and scaling options. Improved grammar.

EDIT 4 Updates from Maverick and minor improvements

Comments are too small, so here are some thoughts:

1. Load average is basically how many processes are running on or waiting for CPU attention. For a perfectly loaded system with 1 CPU core, the load average should be 1.0; for a 4 core system, it should be 4.0. The moment you run the web test, the threading rockets and you have a lot of processes waiting for CPU. Unless the load average exceeds the number of CPU cores by a significant margin, it is not a concern
2. The first 'Time per request' value of 4s correlates to the length of the request queue - 1000 requests dumped on Django nearly instantaneously and took on average 4s to service, about 3.4s of which were waiting in a queue. This is due to the very heavy mismatch between the number of requests (100) vs. the number of processors (16) causing 84 of the requests to be waiting for a processor at any one moment.

3. Running at a concurrency of 100, the tests take 41 seconds at 24 requests/sec. You have 16 processes (threads), so each request is processed about 700ms. Given your type of transaction, that is a long time per request. This may be because:

   1. The CPU cost of each request is high in Django (which is highly unlikely given the low CPU value from the debug toolbar)
   2. The OS is task switching a lot (especially if the load average is higher than 4-8), and the latency is purely down to having too many processes.
   3. There are not enough DB connections serving the 16 processes so processes are waiting to have one come available. Do you have at least one connection available per process?

   4. There is considerable latency around the DB, either:

      1. Tens of small requests each taking, say, 10ms, most of which is networking overhead. If so, can you introducing caching or reduce the SQL calls to a smaller number. Or
      2. One or a couple of requests are taking 100's of ms. To check this, run profiling on the DB. If so, you need to optimise that request.

4. The split between system and user CPU cost is unusually high in system, although the total CPU is low. This implies that most of the work in Django is kernel related, such as networking or disk. In this scenario, it might be network costs (eg receiving and sending HTTP requests and receiving and sending requests to the DB). Sometimes this will be high because of paging. If there's no paging going on, then you probably don't have to worry about this at all.

5. You have set the processes at 16, but have a high load average (how high you don't state). Ideally you should always have at least one process waiting for CPU (so that CPUs don't spin idly). Processes here don't seem CPU bound, but have a significant latency, so you need more processes than cores. How many more? Try running the uwsgi with different numbers of processors (1, 2, 4, 8, 12, 16, 24, etc) until you have the best throughput. If you change latency of the average process, you will need to adjust this again.
6. The 500 concurrency level definitely is a problem, but is it the client or the server? The report says 50 (out of 100) had the incorrect content-length which implies a server problem. The non-2xx also seems to point there. Is it possible to capture the non-2xx responses for debugging - stack traces or the specific error message would be incredibly useful (EDIT) and is caused by the uwsgi request queue running with it's default value of 100.

So, in summary:

1. Django seems fine
2. Mismatch between concurrency of load test (100 or 500) vs. processes (16): You're pushing way too many concurrent requests into the system for the number of processes to handle. Once you are above the number of processes, all that will happen is that you will lengthen the HTTP Request queue in the web server

3. There is a large latency, so either

   1. Mismatch between processes (16) and CPU cores (1): If the load average is >3, then it's probably too many processes. Try again with a smaller number of processes

      1. Load average > 2 -> try 8 processes
      2. Load average > 4 -> try 4 processes
      3. Load average > 8 -> try 2 processes

   2. If the load average <3, it may be in the DB, so profile the DB to see whether there are loads of small requests (additively causing the latency) or one or two SQL statements are the problem

4. Without capturing the failed response, there's not much I can say about the failures at 500 concurrency

Developing ideas

Your load averages >10 on a single cored machine is really nasty and (as you observe) leads to a lot of task switching and general slow behaviour. I personally don't remember seeing a machine with a load average of 19 (which you have for 16 processes) - congratulations for getting it so high ;)

The DB performance is great, so I'd give that an all-clear right now.

Paging: To answer you question on how to see paging - you can detect OS paging in several ways. For example, in top, the header has page-ins and outs (see the last line):

Processes: 170 total, 3 running, 4 stuck, 163 sleeping, 927 threads                                                                                                        15:06:31
Load Avg: 0.90, 1.19, 1.94  CPU usage: 1.37% user, 2.97% sys, 95.65% idle  SharedLibs: 144M resident, 0B data, 24M linkedit.
MemRegions: 31726 total, 2541M resident, 120M private, 817M shared. PhysMem: 1420M wired, 3548M active, 1703M inactive, 6671M used, 1514M free.
VM: 392G vsize, 1286M framework vsize, 1534241(0) pageins, 0(0) pageouts. Networks: packets: 789684/288M in, 912863/482M out. Disks: 739807/15G read, 996745/24G written.

Number of processes: In your current configuration, the number of processes is way too high. Scale the number of processes back to a 2. We might bring this value up later, depending on shifting further load off this server.

Location of Apache Benchmark: The load average of 1.85 for one process suggests to me that you are running the load generator on the same machine as uwsgi - is that correct?

If so, you really need to run this from another machine otherwise the test runs are not representative of actual load - you're taking memory and CPU from the web processes for use in the load generator. In addition, the load generator's 100 or 500 threads will generally stress your server in a way that does not happen in real life. Indeed this might be the reason the whole test fails.

Location of the DB: The load average for one process also suggest that you are running the DB on the same machine as the web processes - is this correct?

If I'm correct about the DB, then the first and best way to start scaling is to move the DB to another machine. We do this for a couple of reasons:

1. A DB server needs a different hardware profile from a processing node:

   1. Disk: DB needs a lot of fast, redundant, backed up disk, and a processing node needs just a basic disk
   2. CPU: A processing node needs the fastest CPU you can afford whereas a DB machine can often make do without (often its performance is gated on disk and RAM)
   3. RAM: a DB machine generally needs as much RAM as possible (and the fastest DB has all its data in RAM), whereas many processing nodes need much less (yours needs about 20MB per process - very small
   4. Scaling: Atomic DBs scale best by having monster machines with many CPUs whereas the web tier (not having state) can scale by plugging in many identical small boxen.

2. CPU affinity: It's better for the CPU to have a load average of 1.0 and processes to have affinity to a single core. Doing so maximizes the use of the CPU cache and minimizes task switching overheads. By separating the DB and processing nodes, you are enforcing this affinity in HW.

500 concurrency with exceptions The request queue in the diagram above is at most 100 - if uwsgi receives a request when the queue is full, the request is rejected with a 5xx error. I think this was happening in your 500 concurrency load test - basically the queue filled up with the first 100 or so threads, then the other 400 threads issued the remaining 900 requests and received immediate 5xx errors.

To handle 500 requests per second you need to ensure two things:

1. The Request Queue size is configured to handle the burst: Use the --listen argument to uwsgi
2. The system can handle a throughput at above 500 requests per second if 500 is a normal condition, or a bit below if 500 is a peak. See scaling notes below.

I imagine that uwsgi has the queue set to a smaller number to better handle DDoS attacks; if placed under huge load, most requests immediately fail with almost no processing allowing the box as a whole to still be responsive to the administrators.

General advice for scaling a system

Your most important consideration is probably to maximize throughput. Another possible need to minimize response time, but I won't discuss this here. In maximising throughput, you are trying to maximize the system, not individual components; some local decreases might improve overall system throughput (for example, making a change that happens to add latency in the web tier in order to improve performance of the DB is a net gain).

Onto specifics:

1. Move the DB to a separate machine. After this, profile the DB during your load test by running top and your favorite MySQL monitoring tool. You need to be able to profile . Moving the DB to a separate machine will introduce some additional latency (several ms) per request, so expect to slightly increase the number of processes at the web tier to keep the same throughput.
2. Ensure that uswgi request queue is large enough to handle a burst of traffic using the --listen argument. This should be several times the maximum steady-state requests-per-second your system can handle.

3. On the web/app tier: Balance the number of processes with the number of CPU cores and the inherent latency in the process. Too many processes slows performance, too few means that you'll never fully utilize the system resources. There is no fixed balancing point, as every application and usage pattern is different, so benchmark and adjust. As a guide, use the processes' latency, if each task has:

   • 0% latency, then you need 1 process per core
   • 50% latency (i.e. the CPU time is half the actual time), then you need 2 processes per core
   • 67% latency, then you need 3 processes per core

4. Check top during the test to ensure that you are above 90% cpu utilisation (for every core) and you have a load average a little above 1.0. If the load average is higher, scale back the processes. If all goes well, at some point you won't be able to achieve this target, and DB might now be the bottleneck

5. At some point you will need more power in the web tier. You can either choose to add more CPU to the machine (relatively easy) and so add more processes, and/or you can add in more processing nodes (horizontal scaleability). The latter can be achieved in uwsgi using the method discussed here by Łukasz Mierzwa