client struct: lazy init components and optimize struct layout

[uriyage]

Refactor client structure to use modular data components

Current State

The client structure allocates memory for replication / pubsub / multi-keys / module / blocked data for every client, despite these features being used by only a small subset of clients. In addition the current field layout in the client struct is suboptimal, with poor alignment and unnecessary padding between fields, leading to a larger than necessary memory footprint of 896 bytes per client. Furthermore, fields that are frequently accessed together during operations are scattered throughout the struct, resulting in poor cache locality.

This PR's Change

1. Lazy Initialization

• Components are only allocated when first used:
  • PubSubData: Created on first SUBSCRIBE/PUBLISH operation
  • ReplicationData: Initialized only for replica connections
  • ModuleData: Allocated when module interaction begins
  • BlockingState: Created when first blocking command is issued
  • MultiState: Initialized on MULTI command

2. Memory Layout Optimization:

   • Grouped related fields for better locality
   • Moved rarely accessed fields (e.g., client->name) to struct end
   • Optimized field alignment to eliminate padding

3. Additional changes:

   • Moved watched_keys to be static allocated in the mstate struct
   • Relocated replication init logic to replication.c

Key Benefits

• Efficient Memory Usage:
• 45% smaller base client structure - Basic clients now use 528 bytes (down from 896).
• Better memory locality for related operations
• Performance improvement in high throughput scenarios. No performance regressions in other cases.

Performance Impact

Tested with 650 clients and 512 bytes values.

Single Thread Performance

Operation	Dataset	New (ops/sec)	Old (ops/sec)	Change %
SET	1 key	261,799	258,261	+1.37%
SET	3M keys	209,134	~209,000	~0%
GET	1 key	281,564	277,965	+1.29%
GET	3M keys	231,158	228,410	+1.20%

8 IO Threads Performance

Operation	Dataset	New (ops/sec)	Old (ops/sec)	Change %
SET	1 key	1,331,578	1,331,626	-0.00%
SET	3M keys	1,254,441	1,152,645	+8.83%
GET	1 key	1,293,149	1,289,503	+0.28%
GET	3M keys	1,152,898	1,101,791	+4.64%

Pipeline Performance (3M keys)

Operation	Pipeline Size	New (ops/sec)	Old (ops/sec)	Change %
SET	10	548,964	538,498	+1.94%
SET	20	606,148	594,872	+1.89%
SET	30	631,122	616,606	+2.35%
GET	10	628,482	624,166	+0.69%
GET	20	687,371	681,659	+0.84%
GET	30	725,855	721,102	+0.66%

Observations:

1. Single-threaded operations show consistent improvements (1-1.4%)
2. Multi-threaded performance shows significant gains for large datasets:
   • SET with 3M keys: +8.83% improvement
   • GET with 3M keys: +4.64% improvement
3. Pipeline operations show consistent improvements:
   • SET operations: +1.89% to +2.35%
   • GET operations: +0.66% to +0.84%
4. No performance regressions observed in any test scenario

Related issue:#761

[codecov]

Codecov Report

Attention: Patch coverage is 87.87313% with 65 lines in your changes missing coverage. Please review.

Project coverage is 70.84%. Comparing base (f20d629) to head (618c9eb).

Files with missing lines	Patch %	Lines
src/module.c	4.08%	47 Missing ⚠️
src/replication.c	94.55%	11 Missing ⚠️
src/networking.c	93.54%	4 Missing ⚠️
src/rdb.c	50.00%	2 Missing ⚠️
src/blocked.c	98.38%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1405      +/-   ##
============================================
+ Coverage     70.81%   70.84%   +0.03%     
============================================
  Files           118      118              
  Lines         63561    63596      +35     
============================================
+ Hits          45010    45056      +46     
+ Misses        18551    18540      -11     

Files with missing lines	Coverage Δ
src/acl.c	88.83% <100.00%> (ø)
src/aof.c	80.21% <100.00%> (+0.08%)	⬆️
src/cluster.c	88.51% <100.00%> (ø)
src/cluster_legacy.c	86.67% <100.00%> (-0.04%)	⬇️
src/multi.c	97.30% <100.00%> (+0.87%)	⬆️
src/pubsub.c	96.77% <100.00%> (-0.27%)	⬇️
src/script.c	87.69% <100.00%> (+0.04%)	⬆️
src/server.c	87.42% <100.00%> (+0.02%)	⬆️
src/server.h	100.00% <ø> (ø)
src/timeout.c	90.00% <100.00%> (ø)
... and 6 more

... and 7 files with indirect coverage changes

[zuiderkwast]

• 45% smaller base client structure - Basic clients now use 528 bytes (down from 896).

Amazing!