nntp: reduce memory overhead of zlib

Using Z_FULL_FLUSH at the right places in our event loop, it
appears we can share a single zlib deflate context across ALL
clients in a process.

The zlib deflate context is the biggest factor in per-client
memory use, so being able to share that across many clients
results in a large memory savings.

With 10K idle-but-did-something NNTP clients connected to a
single process on a 64-bit system, TLS+DEFLATE used around
1.8 GB of RSS before this change.  It now uses around 300 MB.
TLS via IO::Socket::SSL alone uses <200MB in the same situation,
so the actual memory reduction is over 10x.

This makes compression less efficient and bandwidth increases
around 45% in informal testing, but it's far better than no
compression at all.  It's likely around the same level of
compression gzip gives on the HTTP side.

Security implications with TLS?  I don't know, but I don't
really care, either...  public-inbox-nntpd doesn't support
authentication and it's up to the client to enable compression.
It's not too different than Varnish caching gzipped responses
on the HTTP side and having responses go to multiple HTTPS
clients.

1 files changed, 37 insertions, 22 deletions

diff --git a/lib/PublicInbox/NNTPdeflate.pm b/lib/PublicInbox/NNTPdeflate.pm
index 66210bfa..78da2a58 100644
--- a/lib/PublicInbox/NNTPdeflate.pm
+++ b/lib/PublicInbox/NNTPdeflate.pm
@@ -2,13 +2,18 @@
 # License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>
 
 # RFC 8054 NNTP COMPRESS DEFLATE implementation
-# Warning, enabling compression for C10K NNTP clients is rather
-# expensive in terms of memory use.
 #
 # RSS usage for 10K idle-but-did-something NNTP clients on 64-bit:
-#   TLS + DEFLATE :  1.8 GB  (MemLevel=9, 1.2 GB with MemLevel=8)
-#   TLS only      :  <200MB
-#   plain         :   <50MB
+#   TLS + DEFLATE[a] :  1.8 GB  (MemLevel=9, 1.2 GB with MemLevel=8)
+#   TLS + DEFLATE[b] :  ~300MB
+#   TLS only         :  <200MB
+#   plain            :   <50MB
+#
+# [a] - initial implementation using per-client Deflate contexts and buffer
+#
+# [b] - memory-optimized implementation using a global deflate context.
+#       It's less efficient in terms of compression, but way more
+#       efficient in terms of server memory usage.
 package PublicInbox::NNTPdeflate;
 use strict;
 use warnings;
@@ -23,11 +28,11 @@ my %IN_OPT = (
         -AppendOutput => 1,
 );
 
-my %OUT_OPT = (
+# global deflate context and buffer
+my $zbuf = \(my $buf = '');
+my $zout = Compress::Raw::Zlib::Deflate->new(
         # nnrpd (INN) and Compress::Raw::Zlib favor MemLevel=9,
-        # but the zlib C library and git use MemLevel=8
-        # as the default.  Using 8 drops our memory use with 10K
-        # TLS clients from 1.8 GB to 1.2 GB, but...
+        # but the zlib C library and git use MemLevel=8 as the default.
         # FIXME: sometimes clients fail with 8, so we use 9
         # -MemLevel => 9,
 
@@ -43,7 +48,6 @@ sub enable {
         unlock_hash(%$self);
         bless $self, $class;
         $self->{zin} = [ Compress::Raw::Zlib::Inflate->new(%IN_OPT), '' ];
-        $self->{zout} = [ Compress::Raw::Zlib::Deflate->new(%OUT_OPT), '' ];
 }
 
 # overrides PublicInbox::NNTP::compressed
@@ -74,31 +78,42 @@ sub do_read ($$$$) {
 # override PublicInbox::DS::msg_more
 sub msg_more ($$) {
         my $self = $_[0];
-        my $zout = $self->{zout};
 
         # $_[1] may be a reference or not for ->deflate
-        my $err = $zout->[0]->deflate($_[1], $zout->[1]);
+        my $err = $zout->deflate($_[1], $zbuf);
         $err == Z_OK or die "->deflate failed $err";
         1;
 }
 
-# SUPER is PublicInbox::DS::write, so $_[1] may be a reference or not
+sub zflush ($) {
+        my ($self) = @_;
+
+        my $deflated = $zbuf;
+        $zbuf = \(my $next = '');
+
+        my $err = $zout->flush($deflated, Z_FULL_FLUSH);
+        $err == Z_OK or die "->flush failed $err";
+
+        # We can still let the lower socket layer do buffering:
+        PublicInbox::DS::msg_more($self, $$deflated);
+}
+
+# compatible with PublicInbox::DS::write, so $_[1] may be a reference or not
 sub write ($$) {
         my $self = $_[0];
-        return $self->SUPER::write($_[1]) if ref($_[1]) eq 'CODE';
-        my $zout = $self->{zout};
-        my $deflated = pop @$zout;
+        return PublicInbox::DS::write($self, $_[1]) if ref($_[1]) eq 'CODE';
+
+        my $deflated = $zbuf;
+        $zbuf = \(my $next = '');
 
         # $_[1] may be a reference or not for ->deflate
-        my $err = $zout->[0]->deflate($_[1], $deflated);
+        my $err = $zout->deflate($_[1], $deflated);
         $err == Z_OK or die "->deflate failed $err";
-        $err = $zout->[0]->flush($deflated, Z_PARTIAL_FLUSH);
+        $err = $zout->flush($deflated, Z_FULL_FLUSH);
         $err == Z_OK or die "->flush failed $err";
 
-        # PublicInbox::DS::write puts partial writes into another buffer,
-        # so we can prepare the next deflate buffer:
-        $zout->[1] = '';
-        $self->SUPER::write(\$deflated);
+        # We can still let the socket layer do buffering:
+        PublicInbox::DS::write($self, $deflated);
 }
 
 1;