Re: [tip:timers/core] [posix] 1535cb8028: stress-ng.epoll.ops_per_sec 36.2% regression

[Mateusz Guzik]

On Thu, Mar 27, 2025 at 2:17 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> On Thu, Mar 27, 2025 at 2:14 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, Mar 27 2025 at 12:37, Eric Dumazet wrote:
> > > On Thu, Mar 27, 2025 at 11:50 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > >> Cute. How much bloat does it cause?
> > >
> > > This would expand 'struct ucounts' by 192 bytes on x86, if the patch
> > > was actually working :)
> > >
> > > Note sure if it is feasible without something more intrusive like
> >
> > I'm not sure about the actual benefit. The problem is that parallel
> > invocations which access the same ucount still will run into contention
> > of the cache line they are modifying.
> >
> > For the signal case, all invocations increment rlimit[SIGPENDING], so
> > putting that into a different cache line does not buy a lot.
> >
> > False sharing is when you have a lot of hot path readers on some other
> > member of the data structure, which happens to share the cache line with
> > the modified member. But that's not really the case here.
>
> We applications stressing all the counters at the same time (from
> different threads)
>
> You seem to focus on posix timers only :)

Well in that case:
(gdb) ptype /o struct ucounts
/* offset      |    size */  type = struct ucounts {
/*      0      |      16 */    struct hlist_node {
/*      0      |       8 */        struct hlist_node *next;
/*      8      |       8 */        struct hlist_node **pprev;

                                   /* total size (bytes):   16 */
                               } node;
/*     16      |       8 */    struct user_namespace *ns;
/*     24      |       4 */    kuid_t uid;
/*     28      |       4 */    atomic_t count;
/*     32      |      96 */    atomic_long_t ucount[12];
/*    128      |     256 */    struct {
/*      0      |       8 */        atomic_long_t val;
                               } rlimit[4];

                               /* total size (bytes):  384 */
                             }

This comes from malloc. Given 384 bytes of size it is going to be
backed by a 512-byte sized buffer -- that's a clear cut waste of 128
bytes.

It is plausible creating a 384-byte sized slab for kmalloc would help
save memory overall (not just for this specific struct), but that
would require extensive testing in real workloads. I think Google is
in position to do it on their fleet and android? fwiw Solaris and
FreeBSD do have slabs of this size and it does save memory over there.
I understand it is a tradeoff, hence I'm not claiming this needs to be
added. I do claim it does warrant evaluation, but I wont blame anyone
for not wanting to do dig into it.

The other option is to lean into it. In this case I point out the
refcount shares the cacheline with some of the limits and that it
could be moved to a dedicated line while still keeping the struct <
512 bytes, thus not spending more memory on allocation. the refcount
changes less frequently than limits themselves so it's not a big deal,
but it can be adjusted "for free" if you will.

while here I would probably change the name of the field. A reference
counter named "count" in a struct named "ucounts", followed by an
"ucount" array is rather unpleasing. How about s/count/refcount?

-- 
Mateusz Guzik <mjguzik gmail.com>