On Thu, Apr 22, 2021 at 2:59 PM Kefu Chai <kchai@redhat.com> wrote:
>
>
>
> On Wed, Apr 21, 2021 at 5:48 PM Ilya Dryomov <idryomov@gmail.com> wrote:
>>
>> On Wed, Apr 21, 2021 at 9:59 AM Kefu Chai <kchai@redhat.com> wrote:
>> >
>> > hi folks,
>> >
>> > while looking at https://github.com/ceph/ceph/pull/32422, i think a probably safer approach is to make the monitor more efficient. currently, monitor is sort of a single-threaded application. quite a few critical code paths of monitor are protected by Monitor::lock, among other things
>> >
>> > - periodical task performed by tick() which is in turn called by SafeTimer. the "safty" of the SafeTimer is ensured by Monitor::lock
>> > - Monitor::_ms_dispatch is also called with the Monitor::lock acquired. in the case of https://github.com/ceph/ceph/pull/32422, one or more kcephfs clients are even able to slow down the whole cluster by asking for the latest osdmap with an ancient one in its hand, if the cluster is able to rebalance/recover in speedy way and accumulate lots of osdmap in a short time.
>> >
>> > a typical scaring use case is:
>> >
>> > 1. an all-flash cluster just completes a rebalance/recover. the rebalance completed quickly, and it leaves the cluster with a ton of osdmaps before some of the clients have a chance to pick up these updated maps.
>> > 2. (kcephfs) clients with ancient osdmaps in their hands wake up randomly, and they want the latest osdmap!
>> > 3. monitors are occupied with loading the maps from rocksdb and encoding them in very large batches (when discussing with the author of https://github.com/ceph/ceph/pull/32422, he mentioned that the total size of inc osdmap could be up to 200~300 MiB).
>> > 4. and the cluster is basically unresponsive.
>> >
>> > so, does it sound like a right way to improve its performance when serving the CPU intensive workload by dissecting the data dependencies in the monitor and to explore the possibility to make the monitor more multi-threaded?
>>
>> Another thing to explore in addition to making the monitor more
>> efficient might be the concept of osdmap for clients as opposed to
>> a full osdmap.  The osdmap already has a client section and an OSD
>> section (everything else, really), but there is no way to indicate
>> interest in just the client section so we always encode the entire
>> thing.  The kernel client for example doesn't even decode the OSD
>> section so a good chunk of CPU cycles spent on encoding is wasted.
>>
>> There are a couple of fields in the OSD section that can be of
>> interest to clients.  One example is require_osd_release and the
>> features of the OSD -- used in Objecter::_calc_target() for an
>> optimization.  If we identify these and move or duplicate them
>> in the client section, implementing client-section-only osdmap
>> subscription should be pretty easy and would save both CPU cycles
>> and network bandwidth.
>
>
> thank you Ilya. to understand which portion the client is interested, i am looking at osdmap_decode() in net/ceph/osdmap.c, so they are:
>
> ===== client-used data =======
> + fsid, epoch, created, modified
> + pools:
>   + type, size, crush_ruleset, object_hash, pg_num, pgp_num
>   - lpg_num, lpgp_num, last_change, snap_seq, snap_epoch, snaps, removed_snaps, auid
>   + flags
>   - crash_replay_interval // we always encode 0 here, BTW
>   + min_size
>   - quota_max_bytes, quota_max_objects
>   - tiers, tier_of,
>   + read_tier, write_tier
>   - properties
>   - hit_set_params, hit_set_period, hit_set_count,
>   - stripe_width
>   - target_max_bytes, target_max_objects, cache_target_dirty_ratio_micro, cache_target_full_ratio_micro, cache_min_flush_age, cache_min_evict_age,
>   - erasure_code_profile,
>   + last_force_request_resend (last_force_op_resend_preluminous)
>   - min_read_recency_for_promote
>   - expected_num_objects
>   - cache_target_dirty_high_ratio_micro
>   - min_write_recency_for_promote
>   - use_gmt_hitset
>   - fast_read
>   - hit_set_grade_decay_rate, hit_set_search_last_n
>   - opts
>   + last_force_op_resend_prenautilus
>   - ... // ignore the rest
> + pool_names
> + pool_max
> + flags
> + max_osd
> + osd_state
> + osd_weight
> + osd_addrs->client_addrs
> + pg_temp
> + crush
> - erasure_code_profiles
> + pg_upmap, pg_upmap_items
> - crush_version, new_removed_snaps, new_purged_snaps, last_up_change, last_in_change
>
> ===== osd-specific data =====
> ....
> * osd_xinfo
>   * features
> * require_osd_release
>
> in which,
>
> - "+" implies the bits that decoded by the kernel
> - "-" implies the bits ignored by the kernel
> - "*" implies the bits that the client would be interested in
>
> so, in other words, we could introduce a variant of osdmap which *only* includes the "+" and "*" fields. and allow the client to subscribe to the variant. probably we can do this with 2 phases:

Hi Kefu,

I don't think basing it on the kernel client alone is good idea.
It's certainly a good start, but I think we are likely to over-discard
because the kernel client omits quite a lot of things.

>
> the first phase:
>
> 0. let OSDMonitor discriminate between different peers by checking the peer's entity type, if the peer is a client it sends the client-used portion of osdmap, otherwise it sends the full version.

I think discriminating by entity type wouldn't work.  For example, the
"ceph" tool would be a "client", but it does need to show and be able to
modify stuff in the OSD section.

>
> the second phase:
>
> 1. add a new feature named FEATURE_OSDMAP_CLIENT to ceph::features::mon, so the monitors in quincy will be able to serve a new variant osdmap
> 2. introduce yet another "what" in MMonSubscribe. like an extension of "osdmap", so in addition to the all-in-one "osdmap", monitor can serve the maps optimized for client-side, and serve subscription of "osdmap.c", where "c" stands for "client", and it should send the full and inc map only contains the "+" and "*" bits listed above.
> 3. OSDMonitor will be able to encode the client-only osdmaps on demand for the "osdmap.c" subscribers.
> 4. if the monmap implies that the monitor supports the FEATURE_OSDMAP_CLIENT feature, it sends "osdmap.c" MMonSubscribe to monitor for the client-only osdmap.

Yeah, something along these lines.  Opt-in subscription independent of
the entity type.

Thanks,

                Ilya