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Setting Nginx FastCGI response buffer sizes.

Nginx FastCGI response buffer sizes

By default when Nginx starts receiving a response from a FastCGI backend (such as PHP-FPM) it will buffer the response in memory before delivering it to the client. Any response larger than the set buffer size is saved to a temporary file on disk.

This process is outlined at the Nginx ngx_http_fastcgi_module page manual page.


Since disk is slow and memory is fast the aim is to get as many FastCGI responses passing only through memory. On the flip side we don't want to set an excessively large buffer as they are created and sized on a per request basis - it's not shared.

The related Nginx options are:

  • fastcgi_buffering first appeared in Nginx 1.5.6 (1.6.0 stable) and can be used to turn buffering completely on/off. It's on by default.

  • fastcgi_buffer_size is a special buffer space used to hold the first chunk of the FastCGI response, which is going to be the HTTP response headers.

    You shouldn't need to adjust this from the default - even if Nginx defaults to the smallest page size of 4KB (your platform will determine if 4/8k buffer) it should fit your typical HTTP header.

    Note: The one exception I have seen are frameworks that push large amounts of cookie data via the Set-Cookie HTTP header during their user verification/login phase - blowing out the buffer and resulting in a HTTP 500 error. In those instances you will need to increase this buffer to 8k/16k/32k to fully accommodate your largest upstream HTTP header being pushed.

  • fastcgi_buffers controls the number and memory size of buffer segments used for the payload of the FastCGI response. Most, if not all of our tweaking will be around this setting for the remainder of this guide.

Determine actual FastCGI response sizes

By grepping our Nginx access logs we can determine both maximum and average response sizes. The basis of this awk recipe was lifted from here:

$ awk '($9 ~ /200/) { i++;sum+=$10;max=$10>max?$10:max; } END { printf("Maximum: %d\nAverage: %d\n",max,i?sum/i:0); }' access.log

# Maximum: 76716
# Average: 10358

Note: these recipes are going to report on all access requests returning an HTTP 200 code, you might want to split out just FastCGI requests into a separate Nginx access log for reporting, like so (PHP-FPM here):

location ~ "\.php$" {
  fastcgi_index index.php;
  if (!-f $realpath_root$fastcgi_script_name) {
    return 404;

  include /etc/nginx/fastcgi_params;
  fastcgi_pass unix:/run/php5/php-fpm.sock;

  # output just FastCGI requests to it's own Nginx log file
  access_log /var/log/nginx/phpfpmonly-access.log;

With these values in hand we are now much better equipped to set fastcgi_buffers.

Setting the buffer size

The fastcgi_buffers setting takes two values, buffer segment count and memory size, by default it will be:

fastcgi_buffers 8 4k|8k;

So a total of 8 buffer segments at either 4k/8k, which is determined by the platform memory page size. For Debian/Ubuntu Linux that turns out to be 4096 bytes (4K) - so a default total buffer size of 32KB.

Based on the maximum/average response sizes determined above we can now raise/lower these values to suit. I typically keep buffer size at the default (memory page size) and adjust only the buffer segment count to a value for keep the bulk/all responses handled fully in buffer RAM.

The default memory page size (in bytes) for an operating system can be determined by the following command:

$ getconf PAGESIZE

If you response size average tips on the higher side you might want to alternatively lower the buffer segment count and raise the memory size in page size multiples (8k/16k/32k).

Verifying results

We can see how often FastCGI responses are being saved to disk by grepping our Nginx error log(s):

$ cat error.log | grep --extended-regexp "\[warn\].+buffered"

# will return lines like:
YYYY/MM/DD HH:MM:SS [warn] 1234#0: *123456 an upstream response is buffered to a temporary file...

Remember its not necessarily a bad situation to have some larger responses buffered to disk - aim for a balance where only a small portion of your largest responses are handled in this way.

The alternative of ramping up fastcgi_buffers to excessively large number and/or size values to fit all FastCGI responses purely in RAM is something I would strongly recommend against. Unless your Nginx server is receiving only a few concurrent requests at any one moment - you risk exhausting your available system memory.

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magnetikonline commented Aug 21, 2022

I'm not sure if this is the right place to post these questions, but what is the general behavior of this buffer? Does it store the data flushed by PHP until it gets a full response? Or is it implemented to limit serial writes and to maximize throughput?

@ClosetMonkey I'm not 100% of those internal behaviours, but reading through the man page, both in-memory and disk buffers have size limits. My assumption would be these buffers are filled - at that point the response is flushed to the client. You want to ideally minimise those "fill & flush" events/moments.

What are the benefits/consequences if turning off the buffers or limiting them to a small size?

A small/disabled buffer is going to mean excessive pulling and flushing from FastCGI -> Nginx -> Client. This is going to tie up FastCGI and thus an available PHP worker. You really want the push the PHP response over to Nginx (and it's buffers) as quickly as possible - this then frees up an allocated PHP worker to accept then another/next request.

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At just 4 buffers by 4kb to 8kb each connection I would imagine that it's flush is not dependent on the state of the response. I did a quick download of this page and it was over 300kb, which would be enough to fill and empty each of it's buffers multiple times each during the single response.

The man page states that the buffers can be enabled/disabled by passing the response header X-Accel-Buffering: yes and X-Accel-Buffering: no regardless of the current fastcgi_buffering configuration. I think that is pretty slick all things considered. I can see reasons where one may desire different buffering strategies within the same application.

Another interesting couple of directive which may also effect buffering are the limit_rate and it's corresponding response header X-Accel-Limit-Rate, as well as limit_rate_after. They don't directly effect the buffering configuration per se, but controlling the amount that passes through the buffers may cause different effects

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