Re: Questions about OPcache interned strings buffer

On 28.11.2021 at 03:44, Micha wrote: > I have two questions regarding the OPcache interned strings bugger, I > hope this mailing list is the right place to ask them, else feel free to > redirect me to a more fitting place. > > 1. What is the logic behind the assigned strings buffer size and the > actually usable size? When using powers of 2, it seems to be exactly 75% > of it being usable, but for values between powers of 2, the usable > buffer size seems to increase by exactly what was added to the setting, > so that the used percentage increases until the next power of 2 is > reached. So basically it is possible that an increase of the settings > value leads to a decrease of the actually usable strings buffer. I'm > just wondering about what the remaining <=25% are used for, whether > using a power of 2 is preferable, or the opposite to have a batter > ratio. To avoid confusion it may be reasonable to add the <=25% always > to the usage statistics output, so that setting and statistics match. > Also Asked on stackoverflow: > > https://stackoverflow.com/questions/67853338/opcache-interned-strings-buffer-size-lower-than-opcache-interned-strings-buffer•Å]û > :TŒô¸c]¼—
d First, interned_string_buffer is indeed supposed to be a power of two[1]. Then it is important to understand that the HashTable which stores references to the actual strings, is placed at the beginning of the interned string buffer, so the reported buffer_size is the size of the actual string buffer, i.e. always smaller than was has been set as interned_string_buffer. That should be documented in the PHP manual. > 2. I have a PHP8.1 instance where I can increase the interned strings > buffer size as much as I want, it is always nearly fully used based on > the statistics. Currently it's 256 MiB assigned, 100% used with 512 MiB > overall OPcache size ~90% used. This doesn't look reasonable, as the > interned strings buffer is also meant to reduce the actual OPcache usage > (? bundling strings), No. The interned string buffer is not meant to reduce the general OPcache SHM size, but rather to avoid re-allocating strings for each request (which would slow down the processing). > so it looks wrong that it is used with more MiB > than the remaining OPcache. Or is it somehow expected that every smaller > and larger fraction is a string is stored in the interned strings buffer > as long as it is available, so that it basically always fills up? If so, > how can one estimate a good size for ones individual instance? Hard to say. You need to try, and settle on some acceptable value. Note that a full interned string buffer won't stop scripts from executing; they may just run a bit slower. [1] <https://github.com/php/php-src/blob/php-7.4.26/ext/opcache/ZendAccelerator.c#L2599> -- Christoph M. Becker