Java G1垃圾回收：Garbage-First 实现高吞吐量与实时目标

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GarbageFirst是Sun Microsystems公司开发的一款针对多处理器系统且内存容量较大的服务器环境的垃圾回收器。它的设计目标是实现软实时性能，并在高吞吐量的同时，尽可能减少对程序运行的中断。GarbageFirst采用了一种全局并行操作的方式，将全局标记（global marking）与对象的创建和修改（mutation）过程同时进行，这显著减少了因垃圾收集导致的暂停时间，提高了系统的并发性和响应性。其核心特性包括： 1. **并发标记**：GarbageFirst采用并发标记技术，这意味着垃圾收集过程中可以继续执行其他线程，从而确保“收集完整性”，即所有可达的对象都能被正确标记，不会漏掉任何可回收的内存区域。这种并发处理方式提高了整体的系统效率。 2. **识别和准备回收区域**：通过并发标记，GarbageFirst能够实时检测出哪些内存区域已经不再被引用，这些区域被称为“准备回收区域”。这样，当收集时机成熟时，系统就可以开始进行并行的内存清理和压缩（compacting evacuation），进一步释放内存空间。 3. **并行执行内存清理**：为了进一步提高吞吐量，GarbageFirst在多处理器系统上并行执行内存清理过程。这不仅减少了单次清理所需的时间，还使得垃圾收集对整体系统的影响降到最低，保持了系统的稳定性和响应速度。 4. **目标定位**：GarbageFirst特别适合于需要高实时性和高吞吐量的应用场景，如大数据处理、云计算等，它旨在提供一个能在多核环境中高效工作的垃圾收集策略。 5. **语言支持**：这一特性主要应用于Java平台，因为G1 GC（Garbage-First GC）是Java垃圾回收器的一部分，专为了解决大规模并发和内存管理问题而设计。 GarbageFirst是一种创新的垃圾回收算法，它通过并发技术和并行化内存管理来优化大型服务器环境下的内存使用和性能，对于追求高效率、低延迟的现代IT系统至关重要。理解并利用好G1 GC，可以显著提升应用程序在多核环境下运行的稳定性和资源利用率。

empty

R0 R1 R2 R3

RemSet1

R0 R1 R2 R3

RemSet3

(a)

(b)

Figure 1: Remembered Set Operation

If a multithreaded program is running on a multiprocessor

machine, using a sequential garbage collector can create a

performance bottleneck. We therefore strive to parallelize

the operations of an evacuation pause as much as possible.

The ﬁrst step of an evacuation pause is to sequentially

choose the collection set (section 3 details the mechanisms

and heuristics of this choice). Next, the main parallel phase

of the evacuation pause starts. GC threads compete to claim

tasks such as scanning pending log buﬀers to update remem-

bered sets, scanning remembered sets and other root groups

for live objects, and evacuating the live objects. There is no

explicit synchronization among tasks other than ensuring

that each task is performed by only one thread.

The evacuation algorithm is similar to the one reported by

Flood et al. [18]. To achieve fast parallel allocation we use

GCLABs, i.e. thread-local GC allocation buﬀers (similar

to mutator TLABs). Threads allocate an object copy in

their GCLAB and compete to install a forwarding pointer

in the old image. The winner is responsible for copying

the object and scanning its contents. A technique based on

work-stealing [1] provides load balancing.

Figure 1 illustrates the operation of an evacuation pause.

Step A shows the remembered set of a collection set region

R1 being used to ﬁnd pointers into the collection set. As will

be discussed in section 2.6, pointers from objects identiﬁed

as garbage via concurrent marking (object a in the ﬁgure)

are not followed.

2.4 Generational Garbage-First

Generational garbage collection [34, 26] has several ad-

vantages, which a collection strategy ignores at its peril.

Newly allocated objects are usually more likely to become

garbage than older objects, and newly allocated objects are

also more likely to be the target of pointer modiﬁcations,

if only because of initialization. We can take advantage of

both of these properties in Garbage-First in a ﬂexible way.

We can heuristically designate a region as young when it is

chosen as a mutator allocation region. This commits the re-

gion to be a member of the next collection set. In return for

this loss of heuristic ﬂexibility, we gain an important beneﬁt:

remembered set processing is not required to consider mod-

iﬁcations in young regions. Reachable young objects will be

scanned after they are evacuated as a normal part of the

next evacuation pause.

Note that a collection set can contain a mix of young and

non-young regions. Other than the special treatment for

remembered sets described above, both kinds of regions are

treated uniformly.

Garbage-First runs in two modes: generational and pure

garbage-ﬁrst. Generational mode is the default, and is used

for all performance results in this paper.

There are two further “submodes” of generational mode:

evacuation pauses can be fully or partially young. A fully-

young pause adds all (and only) the allocated young regions

to the collection set. A partially-young pause chooses all

the allocated young regions, and may add further non-young

regions, as pause times allow (see section 3.2.1).

2.5 Concurrent Marking

Concurrent marking is an important component of the

system. It provides collector completeness without impos-

ing any order on region choice for collection sets (as, for ex-

ample, the Train algorithm of Hudson and Moss [22] does).

Further, it provides the live data information that allows

regions to be collected in “garbage-ﬁrst” order. This section

describes our concurrent marking algorithm.

We use a form of snapshot-at-the-beginning concurrent

marking [36]. In this style, marking is guaranteed to iden-

tify garbage objects that exist at the start of marking, by

marking a logical “snapshot” of the object graph existing at

that point. Objects allocated during marking are necessar-

ily considered live. But while such objects must be consid-

ered marked, they need not be traced: they are not part of

the object graph that exists at the start of marking. This

greatly decreases concurrent marking costs, especially in a

system like Garbage-First that has no physically separate

young generation treated specially by marking.

2.5.1 Marking Data Structures

We maintain two marking bitmaps, labeled previous and

next. The previous marking bitmap is the last bitmap in

which marking has been completed. The next marking bitmap

may be under construction. The two physical bitmaps swap

logical roles as marking is completed. Each bitmap contains

one bit for each address that can be the start of an ob-

ject. With the default 8-byte object alignment, this means

1 bitmap bit for every 64 heap bits. We use a mark stack

to hold (some of) the gray (marked but not yet recursively

scanned) objects.

2.5.2 Initial Marking Pause/Concurrent Marking

The ﬁrst phase of a marking cycle clears the next marking

bitmap. This is performed concurrently. Next, the initial

marking pause stops all mutator threads, and marks all ob-

jects directly reachable from the roots (in the generational

mode, initial marking is in fact piggy-backed on a fully-

young evacuation pause). Each heap region contains two

top at mark start (TAMS) variables, one for the previous

marking and one for the next. We will refer to these as

the previous and next TAMS variables. These variables are

used to identify objects allocated during a marking phase.

These objects above a TAMS value are considered implic-

itly marked with respect to the marking to which the TAMS

variable corresponds, but allocation is not slowed down by

marking bitmap updates. The initial marking pause iterates

over all the regions in the heap, copying the current value of

top in each region to the next TAMS of that region. Steps

A and D of ﬁgure 2 illustrate this. Steps B and E of this

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