Re: Questions about OPcache interned strings buffer

Hi Christoph,

many thanks for your reply.

If interned_string_buffer is supposed to be a power of two, does
"opcache_get_status" then report wrong values, or is it a bug in PHP
that indeed values between powers of two can be applied (as reported
back by "opcache_get_status")? It is however good to know that powers of
two at least seem to be expected, so that it shouldn't be tried to apply
a different one.

Which kind of strings are actually stored. Is it only hardcoded
variables in the PHP code of type string only, or also variable strings
like outputs of a databases queries, and/or can it be also parts of the
code/syntax itself? And is it exclusively handled internally or is there
a way that scripts can manually add strings?

I couldn't observe any issues with the full strings buffer indeed, all I
was wondering is why it is always full. I guess when returned database
values are stored in variables and then land in the strings buffer, this
would be an explanation, e.g. in a forum or blog with lots of posts when
whole post texts are stored in the strings buffer. I guess in this case
there wouldn't be a reasonable way to keep all those cached and we'd
need to find a size with doesn't fill up too much RAM of the whole
server (prevent regular swapping) and allow/rely on OPcache to
internally keep the most requested strings/post in the buffer after a while.

Am 02.12.2021 um 14:49 schrieb Christoph M. Becker:
> 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
> 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