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

[Mateusz Guzik]

On Thu, Mar 27, 2025 at 2:44 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> On Thu, Mar 27, 2025 at 2:43 PM Mateusz Guzik <mjguzik@xxxxxxxxx> wrote:
> >
> > 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?
>
>
> How many 'struct ucounts' are in use in a typical host ?
>
> Compared to other costs, this seems pure noise to me.

I did not claim this is going to increase memory usage in a significant manner.

I claim regardless of this change a 384-byte slab for kmalloc may be
saving memory and this bit may be enough of an excuse to evaluate it,
should someone be interested.

Apart from that I claim that if the 512-byte is going to be used to
back the 384 bytes used by the struct, the patch can trivially move
the refcount to a dedicated cacheline to avoid some of the bouncing
and still fit in the 512-byte allocation. I see no reason to not do
it.

-- 
Mateusz Guzik <mjguzik gmail.com>