introduce dynamic client reply buffer size - save memory on idle clients (#9822)

Current implementation simple idle client which serves no traffic still
use ~17Kb of memory. this is mainly due to a fixed size reply buffer
currently set to 16kb.

We have encountered some cases in which the server operates in a low memory environments.
In such cases a user who wishes to create large connection pools to support potential burst period,
will exhaust a large amount of memory  to maintain connected Idle clients.
Some users may choose to "sacrifice" performance in order to save memory.

This commit introduce a dynamic mechanism to shrink and expend the client reply buffer based on
periodic observed peak.
the algorithm works as follows:
1. each time a client reply buffer has been fully written, the last recorded peak is updated: 
new peak = MAX( last peak, current written size)
2. during clients cron we check for each client if the last observed peak was:
     a. matching the current buffer size - in which case we expend (resize) the buffer size by 100%
     b. less than half the buffer size - in which case we shrink the buffer size by 50%
3. In any case we will **not** resize the buffer in case:
    a. the current buffer peak is less then the current buffer usable size and higher than 1/2 the
      current buffer usable size
    b. the value of (current buffer usable size/2) is less than 1Kib
    c. the value of  (current buffer usable size*2) is larger than 16Kib
4. the peak value is reset to the current buffer position once every **5** seconds. we maintain a new
   field in the client structure (buf_peak_last_reset_time) which is used to keep track of how long it
   passed since the last buffer peak reset.

### **Interface changes:**
**CIENT LIST** - now contains 2 new extra fields:
rbs= < the current size in bytes of the client reply buffer >
rbp=< the current value in bytes of the last observed buffer peak position >

**INFO STATS** - now contains 2 new statistics:
reply_buffer_shrinks = < total number of buffer shrinks performed >
reply_buffer_expends = < total number of buffer expends performed >

Co-authored-by: Oran Agra <oran@redislabs.com>
Co-authored-by: Yoav Steinberg <yoav@redislabs.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/tests/unit/introspection.tcl b/tests/unit/introspection.tcl
index 33a41ee50..59c96b5ad 100644
--- a/tests/unit/introspection.tcl
+++ b/tests/unit/introspection.tcl
@@ -7,7 +7,7 @@ start_server {tags {"introspection"}} {
 
     test {CLIENT LIST} {
         r client list
-    } {id=* addr=*:* laddr=*:* fd=* name=* age=* idle=* flags=N db=* sub=0 psub=0 multi=-1 qbuf=26 qbuf-free=* argv-mem=* multi-mem=0 obl=0 oll=0 omem=0 tot-mem=* events=r cmd=client|list user=* redir=-1 resp=2*}
+    } {id=* addr=*:* laddr=*:* fd=* name=* age=* idle=* flags=N db=* sub=0 psub=0 multi=-1 qbuf=26 qbuf-free=* argv-mem=* multi-mem=0 rbs=* rbp=* obl=0 oll=0 omem=0 tot-mem=* events=r cmd=client|list user=* redir=-1 resp=2*}
 
     test {CLIENT LIST with IDs} {
         set myid [r client id]
@@ -17,7 +17,7 @@ start_server {tags {"introspection"}} {
 
     test {CLIENT INFO} {
         r client info
-    } {id=* addr=*:* laddr=*:* fd=* name=* age=* idle=* flags=N db=* sub=0 psub=0 multi=-1 qbuf=26 qbuf-free=* argv-mem=* multi-mem=0 obl=0 oll=0 omem=0 tot-mem=* events=r cmd=client|info user=* redir=-1 resp=2*}
+    } {id=* addr=*:* laddr=*:* fd=* name=* age=* idle=* flags=N db=* sub=0 psub=0 multi=-1 qbuf=26 qbuf-free=* argv-mem=* multi-mem=0 rbs=* rbp=* obl=0 oll=0 omem=0 tot-mem=* events=r cmd=client|info user=* redir=-1 resp=2*}
 
     test {CLIENT KILL with illegal arguments} {
         assert_error "ERR wrong number of arguments for 'client|kill' command" {r client kill}