Is apc_store() thread safe?

Question

There is nothing mentioned in the PHP.net manual about thread safety.

We are running Hiphop PHP with nginx server. One server receives requests of the order of 30 millions in a day.

Team members are saying that they had previously observed issues with apc_store() when they used it to add some values across requests to the system. Due to multiple threads running, initiated by nginx it seems, some of the values were getting lost, because of multiple threads concurrently referring to the same value stored in the cache and trying to update them. Hence they switched to apc_inc() in those cases.

My Observations

• I did not find any differences in the values resulted from using these functions.
• The values seemed to tally with the number of logs in the nginx access log file for the same period of time.

This makes me believe both are thread safe. Please correct me if I am wrong.

Update

Here is some probably relevant part from /etc/hiphop/hiphop.hdf file -

Server {
        SourceRoot = /var/cake_1.2.0.6311-beta/app/webroot
        RequestTimeoutSeconds = 10
        RequestMemoryMaxBytes = -1
        DefaultDocument = fam/hiphop.php
        Type = fastcgi
        Port = 9000
        IP = 0.0.0.0
        ThreadCount = 140
        ThreadRoundRobin = false
        ThreadDropCacheTimeoutSeconds = 0
        ThreadJobLIFO = false
        StatCache = false
        AlwaysUseRelativePath = false
        RequestInitDocument = /var/cake_1.2.0.6311-beta/app/webroot/openx/www/delivery/startup.php
        RequestInitFunction = requestStart
        ForceServerNameToHeader = false

        EnableKeepAlive = true
        EnableEarlyFlush = true

        ConnectionTimeoutSeconds = 120

        # shutdown options
        GracefulShutdownWait = 0   # in seconds
        HarshShutdown = true
        EvilShutdown = true
        DanglingWait = 0
        MaxPostSize = 8  # in MB
        LibEventSyncSend = true
        ResponseQueueCount = 0
        DefaultCharsetName = UTF-8

        APC {
                EnableApc = true
                TableType = concurrent
                ExpireOnSets = true
        }

}

But, doesn't look like it answers how Hiphop is configured in terms of APC variable usage - - configured to run as one single process with multiple threads or - configured to run with shared memory for APC.

with reference to https://github.com/petehunt/hiphop-php/blob/master/doc/runtime.memory_model.

Answer 1

Yes, it's thread safe! Apcu use rwlock to promise it's thread safe, U can find this in source code apcu/apc_cache.c.