Contents Telektronikk - Telenor

158
Segment size [byte]
8192
6144
4096
2048
0
0 5000 10000 15000
Time [msec]
(a)
quence, a timer generated acknowledgment
can change the segment flow on the
connection. An example of this effect is
found in Figure 3 (a) which displays a
log (Sparc2, SBA-100/2.2.6) of the size
of the segments on a connection with a
4 kbyte window and a user data size of
4096 bytes. There are three different flow
behaviors on the connection; 4096 byte
segments, fluctuation between 1016 and
3080 byte segments, and fluctuation between
2028 and 2068 byte segments. Initially,
4096 byte segments are transmitted.
After nearly 4 seconds a timer generated
acknowledgment acknowledges all
outstanding bytes and announces a window
size of 3080 bytes. This acknowledgment
is generated before all bytes are
copied out of the socket receive buffer,
and the window is thereby reduced corresponding
to the number of bytes still in
the socket receive buffer. This affects the
size of the following segments which
will fluctuate between 1016 and 3080
bytes. Figure 3 (b) presents the segment
flow before and after this timer generated
acknowledgment which acknowledges
4096 bytes and announces a window of
3080 bytes. A similar chain of events
gets the connection into a fluctuation of
transmitting 2028 and 2026 byte segments.
This shows up as the horizontal
line of the last part of the segment size
graph in Figure 3 (a).
3.3 Host architecture and
network adapters
To establish how the difference in the
Sparc2 and the Sparc10 bus architecture
influences the achievable performance
we measured the time for the send and
receive paths for both architectures.
Using the SBA-100 adapters, the measured
driver times are proportional to the
segment size. The receive times of the
SBA-100 adapter include the time it
[byte]
8192
6144
4096
2048
Figure 3 Timer generated acknowledgments
0
3930 3935 3940 3945
Time [msec]
3950 3955 3960
takes to read the cells from the receive
FIFO on the adapter. The corresponding
send times include the time it takes to
write the cells to the transmit FIFO on
the adapter. Using the SBA-200 adapters
the measurable driver times are more or
less byte independent. Obviously, the
driver times for the DMA-based SBA-
200 adapter do not include the time to
transfer the segment between host memory
and the network adapter memory.
(We do not have an Sbus analyzer.) Figure
4 presents for different segment sizes
for both Sparc2 and Sparc10 the total
send and receive times and the driver
send and receive times as seen from the
host:
- the total send time is the time from the
write call is issued to the driver is finished
processing the outgoing segment,
- the driver send time is the time from
the driver processing starts until it is
finished processing the outgoing segment.
- the total receive time is the time from
the host starts processing the network
hardware interrupt to the return of the
read system call, and
- the driver receive time is the time from
the host starts processing the network
hardware interrupt until the packet has
been inserted into the IP input queue.
Each measurement point is the average
of 1000 samples. A client-server program
was written to control the segment flow
through the sending and receiving end
system. The client issues a request which
is answered by a response from the server.
Both the request segment and the
response segment are of the same size.
The reported send and receive times are
taken as the average of the measured
send and receive times at both the client
and the server. To be able to send single
Timer generated acknowledgment
Window size
Unacknowledged bytes
(b)
segments of sizes up to MSS bytes, we
removed the 4096-byte copy limit of the
socket layer (Section 3.1).
As expected, the receive operation is the
most time-consuming. The total send and
receive processing times of the Sparc10
are shorter for all segment sizes compared
to the Sparc2. However, using the
SBA-100 adapters, the driver processing
times are in general faster on the Sparc2.
(The only exception is for small segments.)
This is due to the fact that the
Sparc10 CPU does not have direct access
to the Sbus. The latency to access onadapter
memory is thereby longer. Thus,
a 4.5 times as powerful CPU does not
guarantee higher performance with programmed
I/O adapters. As mentioned
above, the SBA-200 driver processing
times do not include the moving of data
between the host memory and the network
adapter. The Sparc10 SBA-200
driver send times are longer than the
driver receive times. For Sparc2 it is the
other way round. On transmit the driver
must dynamically set up a vector of
DMA address-length pairs. On receive,
only a length field and a pointer need to
be updated. The Sparc2 must in addition
do an invalidation of cache lines mapping
the pages of the receive buffers,
while the Sparc10 runs a single cache
invalidation routine. The Sparc10 SBA-
200 driver send time is slightly longer
than the corresponding Sparc2 times, as
the Sparc10 sets up DVMA mappings for
the buffers to be transmitted.
The send and receive processing times
reflect the average time to process one
single segment. The processing times of
the receive path do not include the time
from the network interface poses an
interrupt until the interrupt is served by
the network driver. Neither do the times
include the acknowledgment generation
and reception. The numbers are therefore