| Internet-Draft | Application-Layer Traffic Optimization | March 2023 |
| Yao, et al. | Expires 14 September 2023 | [Page] |
Most existing literature on load balancing in data center networks(DCN) focuses on balancing traffic between servers (links), but there is relatively little attention given to balancing traffic at the core-level of individual servers. In this draft, we present a load balancing framework for DCN that is aware of core-level load, in order to address this issue. Specifically, our approach transfers real-time load from CPUs to an L4 load balancer, which then selects a core with lower load based on load information to deliver the data packet. Theoretically, our approach can completely avoid this problem, making the system more stable and enabling higher CPU utilization without overprovisioning.¶
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Current load balancing strategies in data centers primarily focus on link balancing, with the goal of distributing traffic evenly across parallel paths to improve network link utilization and prevent network congestion. Methods such as ECMP [RFC2991]and WCMP[WCMP] use hashing to distribute traffic to different paths. However, these methods do not consider core-level server load balancing, which can lead to actual load imbalances in data center networks where heavy-hitter flows coexist with low-rate flows. Some existing works estimate server load balancing between servers based on traffic, but there are two issues. First, load estimation may have bias, leading to traffic being assigned to heavily-loaded servers. Second, these methods lack granularity at the CPU core level, which may result in the phenomenon of single-core overload within servers. In this paper, we attempt to address these issues by transmitting CPU load to the load balancer, which can obtain global load information and then allocate traffic to servers based on core-level targets.¶
Our solution has the following challenges, in server-side, the first challenge is to smoothly manage the rapidly changing load on the server. The second challenge is to insert load information into data packets and transmit it to the load balancer through the shortest path. Finally, it may require multi-hop routing to reach the destination, and thus ensuring real-time load balancing becomes a critical issue. And in load balancer:, the first challenge for the load balancer is to allocate processing cores to streams based on load information. The second challenge is to accurately deliver colored business streams to the appropriate cores. Finally, the load balancer needs to ensure the consistency of streams while minimizing extreme single-core pressure.¶
CID Core Identifier¶
CPU Central Processing Unit¶
LB Load Balancer¶
NIC Network Interface Card¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14[RFC2119][RFC8174] when, and only when, they appear in all capitals, as shown here.¶
We present the overall framework of our design, as shown in Fig 1. Each server sends internal core load information to the load balancer, and we use smoothed CPU utilization as a measure of load. The server encapsulates the load into a load-aware data packet and passes it to the load balancer through layer two or layer three methods. The load balancer can be a x86-based server running virtual machine or a switch based on programmable ASIC. The load balancer parses the load information carried in the load-aware data packet and maintains k server-CPU pairs with the lowest load using the data structure of a minimum heap. New connections are fairly assigned to these k server-CPU pairs by the load balancer. To ensure consistency of flows, we use a Redirect table in the load balancer to record the state of flows. Data packets that hit table entries are directly forwarded to the corresponding CPU.¶
+----------+-----+-----+ +--------------------+
Flow | Flow ID | DIP | CID | miss | Minimum heap with |
--------------> +----------+-----+-----+ ------>|k least-loaded cores|
| | | | | |
+----------+-----+-----+ +----------+---------+
Redirect table |
| Select core |
v hit for IP flow |
+----------------------+<------------------+
data packet| L4 Load-balancer |data packet
+----------+ +----------+
| | (Tofino/x86) | |
| +------->+--+-^-----------^--+--+<------+ |
| | | | Load-aware| | | |
| | | | packet | | | |
+--v-+---+ +--v-+---+ +--+--v--+ +--+--v--+
| Rack1 | | Rack2 | | Rack3 | | Rack4 |
| +----+ | | +----+ | | +----+ | | +----+ |
| +----+ | | +----+ | | +----+ | | +----+ |
| | | | | | | |
| +----+ | | +----+ | | +----+ | | +----+ |
| +----+ | | +----+ | | +----+ | | +----+ |
| | | | | | | |
+--------+ +--------+ +--------+ +--------+
To smooth the historical loads of each core on the server side, we use exponential smoothing to smooth the CPU utilization obtained each time:¶
Load_n = alpha * Load_get + (1-alpha) * Load_n-1¶
Where Load_get is the load value obtained this time, Load_n-1 is the result of the last calculation, and alpha is the smoothing parameter. With the above formula, we can obtain a smoothed load value Load_n to represent the CPU load at this time.¶
When congestion occurs in the network, the core load information carried by the data packets may become invalid. Therefore, we need to record the timestamp when the data packets are sent from the server in the packets. When the packets arrive at the load balancer, the load balancer calculates the transmission delay of the packets. If the transmission delay is too large, the load balancer will not select that server as the target for traffic delivery (assuming there is only one path from the LB to the server) because the path is already congested. At this point, regardless of whether the server's cores are overloaded or not, no traffic will be assigned to that server.¶
To design the packet message structure to carry load information, it is not necessary to record the load of all cores in the packet. Only the load of the lowest n cores needs to be recorded. We have designed two different solutions to deal with different scenarios. Firstly, when there is a fixed path between the load balancer and the server, source routing can be used to send the packets to the load balancer. The packet message structure is designed as follows，after the Ethernet frame header, we add the SR field, and each time it goes to a switch on the path, the packet is bounced one SR header from the stack and finally arrives at the load balancer, and the packet also contains the source IP, CPU ID, CPU load and timestamp.¶
+-+-+-+-+-+-+-+-+-+-+-+~ 48 bits~-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Mac | +-+-+-+-+-+-+-+-+-+-+-+~ 48 bits~-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Mac | +-+~ 16 bits~-+-+~ 8 bits~-+-+~ 8 bits~-+-+~ 8 bits~-+-+~ 8 bits~-+ | Type | SR1 | SR2 | SR3 | SR4 | +-+-+-+-+~ 32 bits~ -+-+-+-+-+-+--+-+~ 8 bits~-+-+-+~ 8 bits~-+-+-+ | Source IP | CPU ID | CPU Load | +-+-+-+-+-+-+-+-+-+-+-+-+~ 48 bits~-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Timestamp | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Secondly, when multiple hops of non-fixed routing are required between the load balancer and the server, the core ID and core load can be inserted in the IPv6 packet and the packet can be routed to the load balancer. We mark the packet as a load notification packet in the Traffic Class field of IPv6, and fill in the 8-bit CPU ID and 8-bit core load in the Flow Label field, and then route it to the load balancer.¶
The load balancer needs to parse the load notification packets from the servers, extract the load of each core in each server, and maintain an internal minimum heap structure. The load information of the cores that has been obtained within a certain period of time is adjusted through the heap to obtain the k cores with the lowest load. Before the next minimum heap is generated, we allocate the new flows to these k cores with the lowest load through polling.¶
Ordinary L4 load balancers map packets destined for a service with a virtual IP address (VIP) to a pool of servers with multiple direct IP addresses (DIP or DIP pool). In our solution, the load balancer needs to be accurate at the core level. Therefore, we specify the DIP and delivery core directly for the first packet and record the flow identifier (Flow ID), direct IP address (DIP), and core identifier (CID) in a table to ensure flow consistency. In case of extreme situations, such as when a large single flow causes too much pressure on the core, we can reduce the CPU load by using the method of back pressure on the large flow.¶
We need to mark the packets to specify the target core in the destination server. For IP, we can overwrite the original VIP with DIP. For core ID, in order not to overwrite the original information of the user packet, we construct a new packet header and insert it into the user packet to be transmitted to the server NIC. The NIC parses the destination core ID, removes the added packet header, and delivers the restored user packet to the designated core.¶
TBD.¶