This patch adds the row bits to the name of the address mapping
schemes to make it more clear that all the current schemes places the
row bits as the most significant bits.
This patch fixes a number of stats accounting issues in the DRAM
controller. Most importantly, it separates the system interface and
DRAM interface so that it is clearer what the actual DRAM bandwidth
(and consequently utilisation) is.
This patch unifies the request selection across read and write queues
for FR-FCFS scheduling policy. It also fixes the request selection
code to prioritize the row hits present in the request queues over the
selection based on earliest bank availability.
This patch adds a basic adaptive version of the open-page policy that
guides the decision to keep open or close by looking at the contents
of the controller queues. If no row hits are found, and bank conflicts
are present, then the row is closed by means of an auto
precharge. This is a well-known technique that should improve
performance in most use-cases.
This patch removes the untimed while loop in the write scheduling
mechanism and now schedule commands taking into account the minimum
timing constraint. It also introduces an optimization to track write
queue size and switch from writes to reads if the number of write
requests fall below write low threshold.
This patch adds the tRRD parameter to the DRAM controller. With the
recent addition of the actAllowedAt member for each bank, this
addition is trivial.
This patch changes the tXAW constraint so that it is enforced per rank
rather than globally for all ranks in the channel. It also avoids
using the bank freeAt to enforce the activation limit, as doing so
also precludes performing any column or row command to the
DRAM. Instead the patch introduces a new variable actAllowedAt for the
banks and use this to track when a potential activation can occur.
This patch fixes the controller when a write threshold of 100% is
used. Earlier for 100% write threshold no data is written to memory
as writes never get triggered since this corner case is not
considered.
This patch changes the FCFS bit of FR-FCFS such that requests that
target the earliest available bank are picked first (as suggested in
the original work on FR-FCFS by Rixner et al). To accommodate this we
add functionality to identify a bank through a one-dimensional
identifier (bank id). The member names of the DRAMPacket are also
update to match the style guide.
This patch changes the time the controller is woken up to take the
next scheduling decisions. tRAS is now handled in estimateLatency and
doDRAMAccess and we do not need to worry about it at scheduling
time. The earliest we need to wake up is to do a pre-charge, row
access and column access before the bus becomes free for use.
This patch adds an explicit tRAS parameter to the DRAM controller
model. Previously tRAS was, rather conservatively, assumed to be tRCD
+ tCL + tRP. The default values for tRAS are chosen to match the
previous behaviour and will be updated later.
This patch updates the stats to reflect the: 1) addition of the
internal queue in SimpleMemory, 2) moving of the memory class outside
FSConfig, 3) fixing up of the 2D vector printing format, 4) specifying
burst size and interface width for the DRAM instead of relying on
cache-line size, 5) performing merging in the DRAM controller write
buffer, and 6) fixing how idle cycles are counted in the atomic and
timing CPU models.
The main reason for bundling them up is to minimise the changeset
size.
This patch changes the data structure used for the DRAM read, write
and response queues from an STL list to deque. This optimisation is
based on the observation that the size is small (and fixed), and that
the structures are frequently iterated over in a linear fashion.
This patch implements basic write merging in the DRAM to avoid
redundant bursts. When a new access is added to the queue it is
compared against the existing entries, and if it is either
intersecting or immediately succeeding/preceeding an existing item it
is merged.
There is currently no attempt made at avoiding iterating over the
existing items in determining whether merging is possible or not.
This patch gets rid of bytesPerCacheLine parameter and makes the DRAM
configuration separate from cache line size. Instead of
bytesPerCacheLine, we define a parameter for the DRAM called
burst_length. The burst_length parameter shows the length of a DRAM
device burst in bits. Also, lines_per_rowbuffer is replaced with
device_rowbuffer_size to improve code portablity.
This patch adds a burst length in beats for each memory type, an
interface width for each memory type, and the memory controller model
is extended to reason about "system" packets vs "dram" packets and
assemble the responses properly. It means that system packets larger
than a full burst are split into multiple dram packets.
This patch removes the notion of a peer block size and instead sets
the cache line size on the system level.
Previously the size was set per cache, and communicated through the
interconnect. There were plenty checks to ensure that everyone had the
same size specified, and these checks are now removed. Another benefit
that is not yet harnessed is that the cache line size is now known at
construction time, rather than after the port binding. Hence, the
block size can be locally stored and does not have to be queried every
time it is used.
A follow-on patch updates the configuration scripts accordingly.
This patch adds a histogram to track how many bytes are accessed in an
open row before it is closed. This metric is useful in characterising
a workload and the efficiency of the DRAM scheduler. For example, a
DDR3-1600 device requires 44 cycles (tRC) before it can activate
another row in the same bank. For a x32 interface (8 bytes per cycle)
that means 8 x 44 = 352 bytes must be transferred to hide the
preparation time.
This patch adds a frontend and backend static latency to the DRAM
controller by delaying the responses. Two parameters expressing the
frontend and backend contributions in absolute time are added to the
controller, and the appropriate latency is added to the responses when
adding them to the (infinite) queued port for sending.
For writes and reads that hit in the write buffer, only the frontend
latency is added. For reads that are serviced by the DRAM, the static
latency is the sum of the pipeline latencies of the entire frontend,
backend and PHY. The default values are chosen based on having roughly
10 pipeline stages in total at 500 MHz.
In the future, it would be sensible to make the controller use its
clock and convert these latencies (and a few of the DRAM timings) to
cycles.
This patch adds an address mapping scheme where the channel
interleaving takes place on a cache line granularity. It is similar to
the existing RaBaChCo that interleaves on a DRAM page, but should give
higher performance when there is less locality in the address
stream.
This patch changes the slightly ambigious names used for the address
mapping scheme to be more descriptive, and actually spell out what
they do. With this patch we also open up for adding more flavours of
open- and close-type mappings, i.e. interleaving across channels with
the open map.
This check covers a case where a retry is called from the SimpleDRAM
causing a new request to appear before the DRAM itself schedules a
nextReqEvent. By adding this check, the event is not scheduled twice.
This patch fixes a number of small cosmetic issues in the SimpleDRAM
module. The most important change is to move the accounting of
received packets to after the check is made if the packet should be
retried or not. Thus, packets are only counted if they are actually
accepted.
This patch adds support for multi-channel instances of the DRAM
controller model by stripping away the channel bits in the address
decoding. The patch relies on the availiability of address
interleaving and, at this time, it is up to the user to configure the
interleaving appropriately. At the moment it is assumed that the
channel interleaving bits are immediately following the column bits
(smallest sensible interleaving). Convenience methods for building
multi-channel configurations will be added later.
This patch adds a check to ensure that the delay incurred by
the bus is not simply disregarded, but accounted for by someone. At
this point, all the modules do is to zero it out, and no additional
time is spent. This highlights where the bus timing is simply dropped
instead of being paid for.
As a follow up, the locations identified in this patch should add this
additional time to the packets in one way or another. For now it
simply acts as a sanity check and highlights where the delay is simply
ignored.
Since no time is added, all regressions remain the same.
This patch adds two additional scheduling constraints to the DRAM
controller model, to constrain the activation rate. The two metrics
are determine the size of the activation window in terms of the number
of activates and the minimum time required for that number of
activates. This maps to current DDRx, LPDDRx and WIOx standards that
have either tFAW (4 activate window) or tTAW (2 activate window)
scheduling constraints.
This patch changes how the data bus busy time is calculated such that
it is delayed to the actual scheduling time of the request as opposed
to being done as soon as possible.
This patch changes a bunch of statistics, and the stats update is
bundled together with the introruction of tFAW/tTAW and the named DRAM
configurations like DDR3 and LPDDR2.
This patch adds a _curTick variable to an eventq. This variable is updated
whenever an event is serviced in function serviceOne(), or all events upto
a particular time are processed in function serviceEvents(). This change
helps when there are eventqs that do not make use of curTick for scheduling
events.
This patch fixes the draining of the SimpleDRAM controller model. The
controller performs buffering of writes and normally there is no need
to ever empty the write buffer (if you have a fast on-chip memory,
then use it). The patch adds checks to ensure the write buffer is
drained when the controller is asked to do so.
This patch moves the draining interface from SimObject to a separate
class that can be used by any object needing draining. However,
objects not visible to the Python code (i.e., objects not deriving
from SimObject) still depend on their parents informing them when to
drain. This patch also gets rid of the CountedDrainEvent (which isn't
really an event) and replaces it with a DrainManager.
This patch adds an additional level of ports in the inheritance
hierarchy, separating out the protocol-specific and protocl-agnostic
parts. All the functionality related to the binding of ports is now
confined to use BaseMaster/BaseSlavePorts, and all the
protocol-specific parts stay in the Master/SlavePort. In the future it
will be possible to add other protocol-specific implementations.
The functions used in the binding of ports, i.e. getMaster/SlavePort
now use the base classes, and the index parameter is updated to use
the PortID typedef with the symbolic InvalidPortID as the default.
This patch introduces a high-level model of a DRAM controller, with a
basic read/write buffer structure, a selectable and customisable
arbiter, a few address mapping options, and the basic DRAM timing
constraints. The parameters make it possible to turn this model into
any desired DDRx/LPDDRx/WideIOx memory controller.
The intention is not to be cycle accurate or capture every aspect of a
DDR DRAM interface, but rather to enable exploring of the high-level
knobs with a good simulation speed. Thus, contrary to e.g. DRAMSim
this module emphasizes simulation speed with a good-enough accuracy.
This module is merely a starting point, and there are plenty additions
and improvements to come. A notable addition is the support for
address-striping in the bus to enable a multi-channel DRAM
controller. Also note that there are still a few "todo's" in the code
base that will be addressed as we go along.
A follow-up patch will add basic performance regressions that use the
traffic generator to exercise a few well-defined corner cases.
