Can anyone tell me what is the memory usage overhead associated with PHP opcode cache?
I've seen a lot of reviews of opcode cache but all of them only concentrate on the performance increase. I have a small entry level VPS and memory limits are a concern for me.
Most of the memory overhead will come from the opcode cache size. Each opcode cacher has their own default(e.g. 30MB for APC) which you can change through the config file.
Other than the cache size, the actual memory overhead of the cacher itself is negligible.
In todays world: It's neglectible. I think memory consumption was about 50 MB bigger with eAccelerator then it was without when I did my benchmarks.
If you really need the speed but do have headaches that your RAM might be not enough: grab $40 and buy another GIG of RAM for your server ;)
You can set a limit to memory consumption for APC, but that potentially limits its effectiveness.
If you're just using it for silent opcode caching, then it should be fine. Once the memory allotment is full, no new files will be cached, but everything will work as expected. However, the user-space cache functions like apc_store() and apc_fetch() will fail silently and inexplicably if there is no memory available.
This can be tricky to catch and debug since no error is reported and no exception is thrown.
Related
What will happen if I set higher value for a function in php.ini, For example set 2GB to opcache.memory_consumption for a normal e-commerce web application.
opcache.memory_consumption = 2048
Below is my pod resource configuration
resources:
limits:
cpu: "1"
memory: "1Gi"
requests:
cpu: "450m"
memory: "256Mi"
If I set 2GB for OPcache itself. What will happen if it's exceed actual memory limit of the application.
From the PHP manual:
opcache.memory_consumption int
The size of the shared memory storage used by OPcache, in megabytes. The minimum permissible value is "8", which is enforced if a smaller value is set.
OK, so what is the shared memory storage used for, and why might we want more of it? As it happens, Nikita Popov, one of the most important developers of PHP's internals right now, just wrote a blog post about how opcache works. Before going into details of how, he summarises thus:
The primary purposes of opcache is to cache compilation artifacts in shared memory, to avoid the need to recompile PHP scripts on every execution.
So, the memory is used to cache the result of compiling your PHP code to an intermediate representation used internally. The amount of space needed will depend on how much compiled code you're trying to cache at once.
How do we know if it's enough? By running the opcache_get_status() function. If the memory is full, or very nearly full, there's a chance that some of your scripts aren't getting cached because they don't fit in memory. In that case, increasing the configured memory size will improve performance by caching those files.
If the memory already has plenty of space, then increasing the size further won't make any difference, other than preventing the server using that memory for other things.
This may seem like a newbie-ish question but I couldn't find an answer for this on google.
I have a server with 128mb of ram. I was wondering if installing memcache will help reduce serverload, say I sacrifice 5-10mb of ram to memcache. Is this even worth it? And if it is, how would I limit memcache to 5-10mb of ram?
I think you should replace apache with nginx first. This will allow you to reduce memory usage, probably even below 64MB, and also server load.
Then, you can allocate up to 60MB remaining memory to memcached, and it should help to reduce load even further.
I am trying to move from the typical combination of APC (for opcode) and Memcache (for my userdata) to a purely apc based cache. Given my usage structure it makes sense and performance is higher.
But unlike before, where the apc cache was limited to a reasonable size and did not affect my data cache, I am now worried that the opcode cache might grow relatively large so that the sum of opcode cache an datacache exceeds the limit. As far as I understand, this would result in a flush of the total cache.
Is there a way to mimic the behaviour, as if apc was only an opcode cache and limit the opcode cache size to a reasonable limit, leaving the rest for user data?
Or should I try setting lower ttl values for the opcode cache, so it always gets flushed first?
It turned out, that my question is irrelevant. Due to the cache fragmentation problems, the cache is quickly marked as full. (see other posts on stackoverflow). As a consequence apc user cache should only be used selectively.
APC is showing 100% fragmentation. Is this bad?
Does it mean that it's not helping at all? What paths do I go down to improve situation?
Thanks in advance.
In my experience, yes. I had a system where APC was showing 100% fragmentation, and performance was bad. I increased APC's memory limit (to 200 MB in my case -- but we had a lot of code) enough to give it some slack room. Fragmentation dropped to zero, and IIRC, CPU usage on the server dropped by 50%.
Also, make sure you're using the apc.php script that comes with APC to monitor fragmentation/utilization. We've even written a nagios check to watch APC, 'cause we have enough traffic that apache locks up entirely when APC fills up.
Moral of the story: give APC enough memory, and monitor utilization.
Fragmentation means that apc often throws out items from it's cache and adds new ones and has trouble finding large enough continous blocks.
There are two main ways to improve performance then
Give more memory to APC. Ideally APC can store your complete scripts in memory.
use apc.filter in php.ini to filter out files which aren't requested often or change often.
Also using apc_store() with a short time to live is bad, as is overwriting using apc_store() often.
[...] Fragmentation is what hurts performance, not the size of memory per se. But it also seems that fragmentation happens when memory is low [...]
Note also that there seems to be a bug with apc.php's graph: http://pecl.php.net/bugs/bug.php?id=13146
I have websites developed in PHP. Im using Opcode cache.
But because Opcode cache like eAccelerator or APC is cached in RAM, I needs too much RAM.
So Im looking for any project or technique which cache the PHP Opcode in hard disk.
Thanks so much
(my website is not generate money, so Im thinking about cheaper solution)
all op-code caches allow you to configure the maximum size of shared memory used (look for a configuration option with shm - for SHared Memory - in the name, eg. apc.shm_size). so you can control that they don't use too much ram.
some caches also allow you to cache on disk instead/besides of caching in ram:
eAccelerator
the question is if a small amount of shared memory or a disk only cache gains you anything in performance compared to plain php without op-code cache. as always when using a cache, you should benchmark this.