Re: Add the ability to limit the amount of memory that can be allocated to backends. - Mailing list pgsql-hackers

Hi,

On 2023-01-05 13:44:20 -0500, Reid Thompson wrote:
> From 0a6b152e0559a250dddd33bd7d43eb0959432e0d Mon Sep 17 00:00:00 2001
> From: Reid Thompson <jreidthompson@nc.rr.com>
> Date: Thu, 11 Aug 2022 12:01:25 -0400
> Subject: [PATCH 1/2] Add tracking of backend memory allocated to
>  pg_stat_activity
> 
> This new field displays the current bytes of memory allocated to the
> backend process. It is updated as memory for the process is
> malloc'd/free'd. Memory allocated to items on the freelist is included in
> the displayed value.

It doesn't actually malloc/free. It tracks palloc/pfree.


> Dynamic shared memory allocations are included only in the value displayed
> for the backend that created them, they are not included in the value for
> backends that are attached to them to avoid double counting.

As mentioned before, I don't think accounting DSM this way makes sense.


> --- a/src/backend/postmaster/autovacuum.c
> +++ b/src/backend/postmaster/autovacuum.c
> @@ -407,6 +407,9 @@ StartAutoVacLauncher(void)
>  
>  #ifndef EXEC_BACKEND
>          case 0:
> +            /* Zero allocated bytes to avoid double counting parent allocation */
> +            pgstat_zero_my_allocated_bytes();
> +
>              /* in postmaster child ... */
>              InitPostmasterChild();



> @@ -1485,6 +1488,9 @@ StartAutoVacWorker(void)
>  
>  #ifndef EXEC_BACKEND
>          case 0:
> +            /* Zero allocated bytes to avoid double counting parent allocation */
> +            pgstat_zero_my_allocated_bytes();
> +
>              /* in postmaster child ... */
>              InitPostmasterChild();
>  
> diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
> index eac3450774..24278e5c18 100644
> --- a/src/backend/postmaster/postmaster.c
> +++ b/src/backend/postmaster/postmaster.c
> @@ -4102,6 +4102,9 @@ BackendStartup(Port *port)
>      {
>          free(bn);
>  
> +        /* Zero allocated bytes to avoid double counting parent allocation */
> +        pgstat_zero_my_allocated_bytes();
> +
>          /* Detangle from postmaster */
>          InitPostmasterChild();


It doesn't at all seem right to call pgstat_zero_my_allocated_bytes() here,
before even InitPostmasterChild() is called. Nor does it seem right to add the
call to so many places.

Note that this is before we even delete postmaster's memory, see e.g.:
    /*
     * If the PostmasterContext is still around, recycle the space; we don't
     * need it anymore after InitPostgres completes.  Note this does not trash
     * *MyProcPort, because ConnCreate() allocated that space with malloc()
     * ... else we'd need to copy the Port data first.  Also, subsidiary data
     * such as the username isn't lost either; see ProcessStartupPacket().
     */
    if (PostmasterContext)
    {
        MemoryContextDelete(PostmasterContext);
        PostmasterContext = NULL;
    }

calling pgstat_zero_my_allocated_bytes() before we do this will lead to
undercounting memory usage, afaict.


> +/* Enum helper for reporting memory allocated bytes */
> +enum allocation_direction
> +{
> +    PG_ALLOC_DECREASE = -1,
> +    PG_ALLOC_IGNORE,
> +    PG_ALLOC_INCREASE,
> +};

What's the point of this?


> +/* ----------
> + * pgstat_report_allocated_bytes() -
> + *
> + *  Called to report change in memory allocated for this backend.
> + *
> + * my_allocated_bytes initially points to local memory, making it safe to call
> + * this before pgstats has been initialized. allocation_direction is a
> + * positive/negative multiplier enum defined above.
> + * ----------
> + */
> +static inline void
> +pgstat_report_allocated_bytes(int64 allocated_bytes, int allocation_direction)

I don't think this should take allocation_direction as a parameter, I'd make
it two different functions.


> +{
> +    uint64        temp;
> +
> +    /*
> +     * Avoid *my_allocated_bytes unsigned integer overflow on
> +     * PG_ALLOC_DECREASE
> +     */
> +    if (allocation_direction == PG_ALLOC_DECREASE &&
> +        pg_sub_u64_overflow(*my_allocated_bytes, allocated_bytes, &temp))
> +    {
> +        *my_allocated_bytes = 0;
> +        ereport(LOG,
> +                errmsg("Backend %d deallocated %lld bytes, exceeding the %llu bytes it is currently reporting
allocated.Setting reported to 0.",
 
> +                       MyProcPid, (long long) allocated_bytes,
> +                       (unsigned long long) *my_allocated_bytes));

We certainly shouldn't have an ereport in here. This stuff really needs to be
cheap.


> +    }
> +    else
> +        *my_allocated_bytes += (allocated_bytes) * allocation_direction;

Superfluous parens?



> +/* ----------
> + * pgstat_get_all_memory_allocated() -
> + *
> + *    Return a uint64 representing the current shared memory allocated to all
> + *    backends.  This looks directly at the BackendStatusArray, and so will
> + *    provide current information regardless of the age of our transaction's
> + *    snapshot of the status array.
> + *    In the future we will likely utilize additional values - perhaps limit
> + *    backend allocation by user/role, etc.
> + * ----------
> + */
> +uint64
> +pgstat_get_all_backend_memory_allocated(void)
> +{
> +    PgBackendStatus *beentry;
> +    int            i;
> +    uint64        all_memory_allocated = 0;
> +
> +    beentry = BackendStatusArray;
> +
> +    /*
> +     * We probably shouldn't get here before shared memory has been set up,
> +     * but be safe.
> +     */
> +    if (beentry == NULL || BackendActivityBuffer == NULL)
> +        return 0;
> +
> +    /*
> +     * We include AUX procs in all backend memory calculation
> +     */
> +    for (i = 1; i <= NumBackendStatSlots; i++)
> +    {
> +        /*
> +         * We use a volatile pointer here to ensure the compiler doesn't try
> +         * to get cute.
> +         */
> +        volatile PgBackendStatus *vbeentry = beentry;
> +        bool        found;
> +        uint64        allocated_bytes = 0;
> +
> +        for (;;)
> +        {
> +            int            before_changecount;
> +            int            after_changecount;
> +
> +            pgstat_begin_read_activity(vbeentry, before_changecount);
> +
> +            /*
> +             * Ignore invalid entries, which may contain invalid data.
> +             * See pgstat_beshutdown_hook()
> +             */
> +            if (vbeentry->st_procpid > 0)
> +                allocated_bytes = vbeentry->allocated_bytes;
> +
> +            pgstat_end_read_activity(vbeentry, after_changecount);
> +
> +            if ((found = pgstat_read_activity_complete(before_changecount,
> +                                                       after_changecount)))
> +                break;
> +
> +            /* Make sure we can break out of loop if stuck... */
> +            CHECK_FOR_INTERRUPTS();
> +        }
> +
> +        if (found)
> +            all_memory_allocated += allocated_bytes;
> +
> +        beentry++;
> +    }
> +
> +    return all_memory_allocated;
> +}
> +
> +/*
> + * Determine if allocation request will exceed max backend memory allowed.
> + * Do not apply to auxiliary processes.
> + */
> +bool
> +exceeds_max_total_bkend_mem(uint64 allocation_request)
> +{
> +    bool        result = false;
> +
> +    /* Exclude auxiliary processes from the check */
> +    if (MyAuxProcType != NotAnAuxProcess)
> +        return result;
> +
> +    /* Convert max_total_bkend_mem to bytes for comparison */
> +    if (max_total_bkend_mem &&
> +        pgstat_get_all_backend_memory_allocated() +
> +        allocation_request > (uint64) max_total_bkend_mem * 1024 * 1024)
> +    {
> +        /*
> +         * Explicitly identify the OOM being a result of this configuration
> +         * parameter vs a system failure to allocate OOM.
> +         */
> +        ereport(WARNING,
> +                errmsg("allocation would exceed max_total_memory limit (%llu > %llu)",
> +                       (unsigned long long) pgstat_get_all_backend_memory_allocated() +
> +                       allocation_request, (unsigned long long) max_total_bkend_mem * 1024 * 1024));
> +
> +        result = true;
> +    }

I think it's completely unfeasible to execute something as expensive as
pgstat_get_all_backend_memory_allocated() on every allocation. Like,
seriously, no.

And we absolutely definitely shouldn't just add CHECK_FOR_INTERRUPT() calls
into the middle of allocator code.

Greetings,

Andres Freund