Very low memory usage during heavy load test

Question

I have a wordpress website wich I did a heavy load test on. As you can see below the CPU usage is of course high but at the same time the RAM usage never goes higher than 0,4%. The load test doesn't produce any errors but the loading time of the website is very long. I'm worried that the available capacity of the RAM is not really used. Or is that low RAM usage normal for 10000 clients/second?

Server specs:

CPU: AMD Ryzen 7 PRO 3700 8-Core Processor (16 core(s))
RAM: 64 GB
SSD: 960 GB NVMe
Bandwidth: 1 Gbps
OS: Ubuntu 20.04.4 LTS

No caching activated

I get those results with the top command

top - 14:12:22 up  4:11,  2 users,  load average: 29.49, 16.27, 7.29
Tasks: 467 total, 143 running, 301 sleeping,  23 stopped,   0 zombie
%Cpu(s): 27.6 us, 45.5 sy,  0.0 ni, 24.8 id,  0.0 wa,  0.0 hi,  2.1 si,  0.0 st
MiB Mem :  64306.9 total,  50683.7 free,   7559.5 used,   6063.7 buff/cache
MiB Swap:   7630.0 total,   7630.0 free,      0.0 used.  55881.2 avail Mem

    PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
  37346 nginx     20   0  270356 266728   6364 R  78.1   0.4   2:19.89 nginx
  57982 fibonac+  20   0  355720 153908 105132 S  14.0   0.2   0:03.36 php-fpm
  61772 fibonac+  20   0  353552 150692 105776 S  14.0   0.2   0:01.26 php-fpm
   1524 mysql     20   0 9117616 192712  20468 S  13.6   0.3   1:17.50 mysqld
  57048 fibonac+  20   0  355788 153888 105072 S  13.6   0.2   0:04.08 php-fpm
  59186 fibonac+  20   0  353712 151920 105132 S  13.6   0.2   0:02.86 php-fpm
  61435 fibonac+  20   0  353680 150108 105120 R  13.6   0.2   0:01.66 php-fpm
  57693 fibonac+  20   0  355760 153964 105132 S  13.3   0.2   0:04.53 php-fpm
  59195 fibonac+  20   0  353684 151912 105124 S  13.3   0.2   0:02.86 php-fpm
  51154 fibonac+  20   0  357824 156160 105136 R  12.3   0.2   0:08.10 php-fpm
  51197 fibonac+  20   0  357780 156064 105088 R  12.0   0.2   0:07.05 php-fpm
  58034 fibonac+  20   0  355728 153616 105132 R  11.0   0.2   0:03.17 php-fpm
  51261 fibonac+  20   0  357780 156064 105136 R  10.0   0.2   0:06.74 php-fpm
  52563 fibonac+  20   0  358116 162056 110892 R  10.0   0.2   0:06.62 php-fpm
  57410 fibonac+  20   0  355728 153948 105132 R  10.0   0.2   0:03.86 php-fpm
  59521 fibonac+  20   0  351636 149816 105136 R  10.0   0.2   0:01.98 php-fpm
  60246 fibonac+  20   0  353752 151856 105140 R  10.0   0.2   0:02.70 php-fpm
  51147 fibonac+  20   0  357812 156156 105136 R   9.6   0.2   0:07.93 php-fpm
  51210 fibonac+  20   0  357780 156136 105136 R   9.6   0.2   0:08.08 php-fpm
  51225 fibonac+  20   0  357776 156112 105128 R   9.6   0.2   0:07.47 php-fpm
  51241 fibonac+  20   0  357780 156112 105136 R   9.6   0.2   0:06.95 php-fpm
  51251 fibonac+  20   0  357784 156072 105136 R   9.3   0.2   0:06.92 php-fpm
  51255 fibonac+  20   0  357784 156132 105140 R   9.3   0.2   0:07.43 php-fpm
  57054 fibonac+  20   0  355784 153952 105124 R   9.3   0.2   0:03.91 php-fpm
  57056 fibonac+  20   0  355768 153960 105132 R   9.3   0.2   0:03.85 php-fpm
  59192 fibonac+  20   0  353708 151508 105120 R   9.3   0.2   0:02.73 php-fpm
  61746 fibonac+  20   0  343316 140124 105140 R   9.3   0.2   0:00.74 php-fpm
  51233 fibonac+  20   0  357776 156112 105140 R   9.0   0.2   0:06.65 php-fpm
  51243 fibonac+  20   0  357784 156140 105120 R   9.0   0.2   0:07.97 php-fpm
  55161 fibonac+  20   0  355728 153988 105132 R   9.0   0.2   0:04.28 php-fpm
  57424 fibonac+  20   0  355736 153600 105132 R   9.0   0.2   0:03.30 php-fpm
  59185 fibonac+  20   0  353684 151496 104832 R   9.0   0.2   0:02.73 php-fpm
  51177 fibonac+  20   0  357812 156132 105136 R   8.6   0.2   0:07.17 php-fpm
  57401 fibonac+  20   0  355728 153620 105132 R   8.6   0.2   0:03.25 php-fpm
  51239 fibonac+  20   0  357836 156128 105136 R   8.3   0.2   0:06.93 php-fpm
  54835 fibonac+  20   0  357776 155544 104892 R   8.3   0.2   0:04.93 php-fpm
  61764 fibonac+  20   0  345492 143512 105588 R   8.3   0.2   0:01.07 php-fpm
  50951 fibonac+  20   0  357860 156164 105140 R   8.0   0.2   0:08.77 php-fpm
  51024 fibonac+  20   0  357816 156128 105136 R   8.0   0.2   0:07.97 php-fpm
  51160 fibonac+  20   0  357808 156156 105136 R   8.0   0.2   0:07.45 php-fpm
  54833 fibonac+  20   0  357832 155788 104816 R   8.0   0.2   0:06.09 php-fpm
  57047 fibonac+  20   0  355788 153864 105068 R   8.0   0.2   0:03.91 php-fpm
  59193 fibonac+  20   0  355760 153208 105080 R   8.0   0.2   0:03.08 php-fpm
  63679 fibonac+  20   0  260456 107648  80836 R   8.0   0.2   0:00.24 php-fpm
  63682 fibonac+  20   0  260456 107672  80860 R   8.0   0.2   0:00.24 php-fpm
  51028 fibonac+  20   0  357856 156068 105136 R   7.6   0.2   0:06.58 php-fpm
  53620 fibonac+  20   0  357776 156048 105132 R   7.6   0.2   0:05.68 php-fpm
  54832 fibonac+  20   0  357776 155968 105052 R   7.6   0.2   0:05.61 php-fpm
  55152 fibonac+  20   0  357780 156040 105132 R   7.6   0.2   0:05.36 php-fpm
  57981 fibonac+  20   0  355732 153596 105132 R   7.6   0.2   0:03.30 php-fpm
  58035 fibonac+  20   0  355736 153604 105132 R   7.6   0.2   0:03.29 php-fpm
  59184 fibonac+  20   0  353736 151484 105100 R   7.6   0.2   0:02.47 php-fpm
  59189 fibonac+  20   0  356072 159176 110760 R   7.6   0.2   0:05.38 php-fpm
  63683 fibonac+  20   0  260456 106668  80072 R   7.6   0.2   0:00.23 php-fpm
  51208 fibonac+  20   0  357780 156188 105140 R   7.3   0.2   0:07.25 php-fpm
  51237 fibonac+  20   0  357776 156184 105136 R   7.3   0.2   0:07.43 php-fpm

no caching enabled could ve the name that the memory foot print is such low ;) but whats the issue you are eally facing, in base its a business related question or issue? — djdomi, Apr 10 '22 at 15:09
well, I deactivated caching because some of my pages are very dynamic. Actually I'm not facing any errors, I'm just surprised that the memory usage is so low and I'm worried that my server would be able to process requests faster if it was using the available memory — deepbluecake123, Apr 10 '22 at 15:34
feel free to have more ram as required. dont worry about something that does not exist.... — djdomi, Apr 10 '22 at 19:35
Why do you *think* RAM use should be higher? What should use more RAM in your opinion? — vidarlo, Apr 10 '22 at 21:27
@vidarlo I thought that the HTTP query processing would need more ram. I thought something wasn't configured right so that theres a limit on the RAM or something like that. But if that's not the case well fine then I'm gonna use the free RAM for something else to help humanity. For example a distributed computing project like DreamLab — deepbluecake123, Apr 11 '22 at 15:52

score 2 · Accepted Answer · answered Apr 11 '22 at 00:13

10000 clients/second?

What order of magnitude of requests do you actually expect to get? Stack Exchange, the entire network, peaks at maybe 5,000 requests per second for ~1.3 billion page views per month. A direct comparison is difficult, but I assume what you are doing is smaller than that.

the available capacity of the RAM is not really used.

You are correct that much of your memory is not used at this point in time. 50683 MB free is a lot in both absolute number, and as 78% of 64306 MB total. Whether to treat this is as wasteful, generous allocation for growth, or one size fits most 64 GB servers, is your judgement call, as a part of capacity planning.

Sixty (?) web server processes plus some other odds and ends is no big deal for 64 GB of memory. Notice RES of about 150 MB each. While in theory that could sum to 5,000 MB, memory accounting is always more complicated than the simple assumptions people make. Linux is lazy and tends to allocate physical memory pages for unique data. Dozens of copies of the same task "compresses" well. Especially for computational benchmark workloads, which likely can drive up CPU with a small working set.

With plenty of free memory, but poor response time to an application, there definitely is other limiting factors to performance. Finding it requires a methodical approach to examining all aspects of the system. Anything from php tuning parameters, to other resources (network?), to application concurrency issues.

As a practical matter, on Linux install debug symbols and run perf top Knowing which functions are on CPU can help analyze what is going on deep in user or kernel code. Ideally you also have something like an APM tool that can profile code.

Very low memory usage during heavy load test

1 Answers1