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From "Feldhacker, Chris" <>
Subject [users@httpd] Slow performance of Apache on Windows
Date Wed, 09 Mar 2011 21:15:28 GMT
Apache HTTP Server 2.2.17 running on Windows Server 2003 & 2008 has slight HTTP transfer
performance problems with Windows XP clients, and *extreme* performance problems with Windows
7 clients.  
Apache HTTP Server running on RedHat Linux 5 performs fine with both XP and W7 clients.  
IIS and Small HTTP Server on Windows 2003/2008 perform fine with both XP and W7 clients.

The purpose of this email is simply to report our findings for the benefit of anyone else
that may have encountered similar problems.  (And if anyone has the ability or desire to dive
in and test other clients or scenarios, all the better.  Ideally, it would be great if others
could reproduce and confirm the findings so that this ultimately gets investigated and resolved
with the Apache HTTP server for Windows.)

I know, I know, Apache server can't possibly have problems running on the Windows platform
that causes different clients to perform differently -- it must be something else.  Depending
on your perspective and the tests run, the evidence will point in different directions and
send you in circles.  We're just reporting our findings.

We initially noticed problems using IBM Installation Manager which downloads several GBs of
files from our internal HTTP server (using Apache on Windows Server 2003) in order to install/update
various IBM products.  With XP clients, transfers were fairly fast at 4000-5000KB/s.  With
Win7 clients, transfers were pretty slow at 350-1000KB/s.  HTTP downloads from the internet
seem normal, it seems the problem only occurred when using HTTP against an internal server.
 We were able to reproduce the problem using Curl as the HTTP client, so HTTP client didn't
seem to be the problem. 

Obviously the problem is with the client OS.  Yes, there are widespread reports of network
performance problems with Vista and Windows 7.  Microsoft docs indicate the most common cause
is problematic network devices that don't handle TCP window scaling or other RFC 1323 options
very well, so there are various workarounds to adjust the TCP settings, most noteably the
auto-tuning level.  Network traces confirmed that window scaling and other 1323 flags were
indeed present and being preserved between the client and the server.  Nonetheless, we fully
explored all the Windows 7 TCP global parameters:  rss, chimney offloading, dma, dca, auto-tuning,
congestion control provider, ecn, timestamps.  Nothing resolved the issue.

Network traces taken in an isolated testing lab with desktop clients running on identical
hardware on a 100mbit network also seemed to point to problems with the client OS: after the
server sends a window of packets (44) and the client dutifully acknowledges every other one,
when the last packet is sent with the push flag, the Windows 7 client takes between 100-300ms
longer to send the acknowledgement compared to the Windows XP client.  (Hmm...200ms sounds
like an issue with selective acknowledgements/nagle's algorithm/TCP_NO_DELAY -- nope, explored
that too.)

Again, obviously a problem with the client.  Maybe it's a 64/32-bit issue (nope), maybe it's
a virusscanner or firewall issue (nope and nope), maybe results would be different if we installed
W7 SP1 (nope), removed the QoS provider (nope), or disabled IPv6 (nope).  Thinking it could
be a network driver issue we made sure ALL drivers were up-to-date, and verified the driver
settings were the same between the XP and W7 machines (all the same, W7 has more options but
nothing we tried changing improved performance).  Monitoring the port on the network switch,
we could confirm that duplexing mismatch was not an issue.  Oh, we also disabled some RDC
feature that's present in Windows 7, no luck.

It was still odd that the problem only seemed to occur from our internal server, HTTP downloads
from the internet seemed fine.  Out of curiosity we setup a Windows Server 2008 running Apache,
and a RedHat Linux server running Apache.  The performance of the 2008 server was the same
as the 2003 server, but the performance of the Linux server was double that of the Windows
machines (10-11MB/s)!  In fact, not only did the Windows 7 client maintain 10-11MB/s, but
the Windows XP client had the same 10-11MB/s performance as well!

Obviously it's a problem with the server.  Since Apache was being used across all platforms,
we initially looked at the OS.  Again, we tried changing various registry changes to tweak
the TCP settings (TcpWindowSize both above and below 64K, Tcp1323Opts (0,1,3), SackOpts, RSS,
ICMPRedirect, etc.) -- pretty much everything we could find to no avail.  When we setup a
file share and copied a large file from the Windows server to the clients directly using Explorer,
both the XP and W7 clients performed about the same (roughly 8MB/s).  We installed the Test
TCP (TTCP) tool on the Windows servers to test the raw TCP traffic and both the XP and W7
clients obtained 10-11MB/s.

We were about to open a ticket with Microsoft and see what they had to say, but we wanted
to get to a state where only Microsoft products were involved -- so Microsoft could only blame
themselves.  So, we already experienced this problem with different HTTP clients (IBM IM,
curl), but we went ahead and reproduced the problem with Wget and Firefox and IE8.  (Now,
performance does vary slightly between clients, but no HTTP client climbs above 1MB/s or so
-- certainly far short of the 10-11MB/s that we know is possible!)  But, at least the problem
still occurs with Microsoft IE as the HTTP client.

Next, we installed IIS on the server.  HTTP performance with both XP and W7 clients was now
10-11MB/s, but if we hit against Apache we were still getting 4MB/s with XP and at most 1MB/s
with W7!  Surely our Apache HTTP server must be setup wrong.  But, we just had a default install
of Apache HTTP 2.2.17 with absolutely no changes at all.  It's quite odd that the default
install should perform so poorly, but we started stripping down the config file to make it
was as bare-bones as possible -- removed as many modules and directives as we could; after
all, we were just providing a direct URL to a large file and just wanted it to transfer, nothing
more.  We've tried the platform specific options (EnableSendfile Off, EnableMMAP Off, Win32DisableAcceptEx),
and tweaking the settings for the number of clients, threads, or age doesn't matter since
we're just testing one client and one request.  No luck.

Of course IIS performs better on Windows than Apache, Microsoft must have tweaked it or optimized
it to perform better, right?  The number of alternative HTTP servers for Windows appears to
be quite small, and the only one that I found that I could install and setup quickly was "Small
HTTP Server".  Using Small HTTP Server, HTTP performance with both XP and W7 clients was still
10-11MB/s!  I guess the deck really isn't stacked -- if Small HTTP Server can achieve 10-11MB/s,
then shouldn't Apache be able to achieve this as well?

End result:  We have a Windows 2003 Server with Apache HTTP Server, IIS, and Small HTTP Server
all installed and running.  If a Windows XP client hits IIS or SHTTPS they get 10-11MB/s,
but if they hit Apache it gets 4MB/s.  If a Windows 7 client hits IIS or SHTTPS it also gets
10-11MB/s, but if it hits Apache it gets AT MOST 1MB/s.

Conclusion:  Apache HTTP Server on Windows exhibits performance issues with certain client
OSes -- performance is 2 times as slow for XP clients, and 10-30 times slower for W7 clients.
 I have no explanation as to why.  (And I completely acknowledge that the behavior observed
doesn't even make sense, but it is what it is!)  It seems our Apache/Windows server has had
performance issues this whole time we've been using it and didn't realize it with our XP clients,
it wasn't until we started testing with Windows 7 clients that the performance problems seem
to be amplified.

In our mind we've resolved our issue -- use IIS on Windows Server instead of Apache, or switch
to Apache on Linux.  Exactly why Apache/Windows has performance issues is unresolved and a
lot more investigation could be done, namely testing with additional clients besides just
Windows, more detailed network tracing and analysis, or OS/Apache debugging and troubleshooting.

I apologize for the novel, but depending on where you start, what results you focus on, and
what biases you carry, the evidence will lead you in different directions.  There are definitely
"known" performance issues in the network, client and server OSes that had to be eliminated
as possible causes first.  You really have to get the complete story or spin in circles a
few times on your own before the conclusion becomes clear.

Hopefully the "end result" above can serve as simple "steps to reproduce" and allow other
users to reproduce and confirm the problem quickly and easily.  

I hope somebody else will someday gain some benefit from these findings.

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