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ceragon edited this page Feb 25, 2022 · 5 revisions

参数

-Xms19500M
-Xmx19500M
-XX:+UseG1GC
-XX:+AlwaysPreTouch
-XX:MaxGCPauseMillis=400
-XX:+UnlockExperimentalVMOptions
-XX:G1MaxNewSizePercent=80
-Xlog:gc*=info:file=/usr/local/tomcat/logs/gc.log:time,tags,level
  • region size: 8Mb
  • region max num: 2438
  • _min_heap_byte_size: 20451426304(byte) = 19972096(kb) = 19504(Mb) = 8 * 2438
  • min young length: 2438 * 5% = 121
  • max young length: 2438 * 80% = 1950
  • _reserve_regions: 2438 * 10% = 243.8 = 244(ceil)
  • use regions: 2438 - 244 = 2194

日志

[2022-01-25T19:23:57.286+0000][info][gc,start      ] GC(36) Pause Young (Normal) (G1 Evacuation Pause)
[2022-01-25T19:23:57.287+0000][info][gc,task       ] GC(36) Using 11 workers of 11 for evacuation
[2022-01-25T19:23:57.366+0000][info][gc,phases     ] GC(36)   Pre Evacuate Collection Set: 0.2ms
[2022-01-25T19:23:57.366+0000][info][gc,phases     ] GC(36)   Evacuate Collection Set: 73.3ms
[2022-01-25T19:23:57.366+0000][info][gc,phases     ] GC(36)   Post Evacuate Collection Set: 5.7ms
[2022-01-25T19:23:57.366+0000][info][gc,phases     ] GC(36)   Other: 0.6ms
[2022-01-25T19:23:57.366+0000][info][gc,heap       ] GC(36) Eden regions: 1893->0(1898)
[2022-01-25T19:23:57.366+0000][info][gc,heap       ] GC(36) Survivor regions: 57->52(244)
[2022-01-25T19:23:57.366+0000][info][gc,heap       ] GC(36) Old regions: 64->65
[2022-01-25T19:23:57.366+0000][info][gc,heap       ] GC(36) Humongous regions: 0->0
[2022-01-25T19:23:57.366+0000][info][gc,metaspace  ] GC(36) Metaspace: 114532K->114532K(1153024K)
[2022-01-25T19:23:57.366+0000][info][gc            ] GC(36) Pause Young (Normal) (G1 Evacuation Pause) 16098M->921M(19504M) 79.885ms
[2022-01-25T19:23:57.366+0000][info][gc,cpu        ] GC(36) User=0.86s Sys=0.00s Real=0.08s
[2022-01-25T19:24:12.305+0000][info][gc,start      ] GC(37) Pause Young (Normal) (G1 Evacuation Pause)
[2022-01-25T19:24:12.305+0000][info][gc,task       ] GC(37) Using 11 workers of 11 for evacuation
[2022-01-25T19:24:12.376+0000][info][gc,phases     ] GC(37)   Pre Evacuate Collection Set: 0.2ms
[2022-01-25T19:24:12.376+0000][info][gc,phases     ] GC(37)   Evacuate Collection Set: 63.7ms
[2022-01-25T19:24:12.376+0000][info][gc,phases     ] GC(37)   Post Evacuate Collection Set: 6.2ms
[2022-01-25T19:24:12.376+0000][info][gc,phases     ] GC(37)   Other: 0.6ms
[2022-01-25T19:24:12.376+0000][info][gc,heap       ] GC(37) Eden regions: 1898->0(80)
[2022-01-25T19:24:12.376+0000][info][gc,heap       ] GC(37) Survivor regions: 52->41(244)
[2022-01-25T19:24:12.376+0000][info][gc,heap       ] GC(37) Old regions: 65->65
[2022-01-25T19:24:12.376+0000][info][gc,heap       ] GC(37) Humongous regions: 0->0
[2022-01-25T19:24:12.376+0000][info][gc,metaspace  ] GC(37) Metaspace: 114534K->114534K(1153024K)
[2022-01-25T19:24:12.376+0000][info][gc            ] GC(37) Pause Young (Normal) (G1 Evacuation Pause) 16105M->834M(19504M) 70.736ms
[2022-01-25T19:24:12.376+0000][info][gc,cpu        ] GC(37) User=0.75s Sys=0.00s Real=0.07s
[2022-01-25T19:24:12.980+0000][info][gc,start      ] GC(38) Pause Young (Normal) (G1 Evacuation Pause)
[2022-01-25T19:24:12.980+0000][info][gc,task       ] GC(38) Using 11 workers of 11 for evacuation
[2022-01-25T19:24:13.087+0000][info][gc,phases     ] GC(38)   Pre Evacuate Collection Set: 0.2ms
[2022-01-25T19:24:13.087+0000][info][gc,phases     ] GC(38)   Evacuate Collection Set: 104.9ms
[2022-01-25T19:24:13.087+0000][info][gc,phases     ] GC(38)   Post Evacuate Collection Set: 1.3ms
[2022-01-25T19:24:13.087+0000][info][gc,phases     ] GC(38)   Other: 0.5ms
[2022-01-25T19:24:13.087+0000][info][gc,heap       ] GC(38) Eden regions: 80->0(1933)
[2022-01-25T19:24:13.087+0000][info][gc,heap       ] GC(38) Survivor regions: 41->16(16)
[2022-01-25T19:24:13.087+0000][info][gc,heap       ] GC(38) Old regions: 65->111
[2022-01-25T19:24:13.087+0000][info][gc,heap       ] GC(38) Humongous regions: 0->0
[2022-01-25T19:24:13.087+0000][info][gc,metaspace  ] GC(38) Metaspace: 114534K->114534K(1153024K)
[2022-01-25T19:24:13.087+0000][info][gc            ] GC(38) Pause Young (Normal) (G1 Evacuation Pause) 1474M->1006M(19504M) 107.010ms
[2022-01-25T19:24:13.087+0000][info][gc,cpu        ] GC(38) User=1.16s Sys=0.00s Real=0.10s
[2022-01-25T19:24:13.857+0000][info][gc,start      ] GC(39) Pause Young (Normal) (G1 Evacuation Pause)
[2022-01-25T19:24:13.857+0000][info][gc,task       ] GC(39) Using 11 workers of 11 for evacuation
[2022-01-25T19:24:14.026+0000][info][gc,phases     ] GC(39)   Pre Evacuate Collection Set: 0.2ms
[2022-01-25T19:24:14.026+0000][info][gc,phases     ] GC(39)   Evacuate Collection Set: 166.8ms
[2022-01-25T19:24:14.026+0000][info][gc,phases     ] GC(39)   Post Evacuate Collection Set: 1.5ms
[2022-01-25T19:24:14.026+0000][info][gc,phases     ] GC(39)   Other: 0.5ms
[2022-01-25T19:24:14.026+0000][info][gc,heap       ] GC(39) Eden regions: 105->0(105)
[2022-01-25T19:24:14.026+0000][info][gc,heap       ] GC(39) Survivor regions: 16->16(16)
[2022-01-25T19:24:14.026+0000][info][gc,heap       ] GC(39) Old regions: 111->188
[2022-01-25T19:24:14.026+0000][info][gc,heap       ] GC(39) Humongous regions: 0->0
[2022-01-25T19:24:14.026+0000][info][gc,metaspace  ] GC(39) Metaspace: 114535K->114535K(1153024K)
[2022-01-25T19:24:14.026+0000][info][gc            ] GC(39) Pause Young (Normal) (G1 Evacuation Pause) 1846M->1623M(19504M) 169.080ms
[2022-01-25T19:24:14.026+0000][info][gc,cpu        ] GC(39) User=1.83s Sys=0.00s Real=0.17s
gc times eden num min young num survivor num young gen num old gen num heap use num free list num base_free_regions
36(before) 1893 121-57=64 57 1950 64 2014
36(after) 1898 121-52=69 52 1950 65 2015 2438-52-65=2321
37(before) 1898 121-52=69 52 1950 65 2015 2438-52-65=2321 2321-244=2087
37(after) 80 121-41=80 41 121 65 186 2438-41-65=2332 2332-244=2088
38(before) 80 121-41=80 41 121 65 186
38(after) 1933 121-16=105 16 1949 111 2060
39(before) 105 121-16=105 16 121 111 232
39(after) 105 121-16=105 16 121 188 309

young gen 变为最小值的条件

条件一

uint G1Policy::calculate_young_list_target_length(size_t rs_lengths,
                                             uint base_min_length,
                                             uint desired_min_length,
                                             uint desired_max_length) const {
    if (desired_max_length <= desired_min_length) {
        return desired_min_length;
    }
}

desired_max_length 的赋值

void G1YoungGenSizer::adjust_max_new_size(uint number_of_heap_regions) {
    uint temp = _min_desired_young_length;
    uint result = _max_desired_young_length;
    recalculate_min_max_young_length(number_of_heap_regions, &temp, &result);
}

计算可知，desired_max_length == 2438 * 80% = 1950

uint G1YoungGenSizer::calculate_default_max_length(uint new_number_of_heap_regions) {
    uint default_value = (new_number_of_heap_regions * G1MaxNewSizePercent) / 100;
    return MAX2(1U, default_value);
}

条件二

uint G1Policy::calculate_young_list_target_length(size_t rs_lengths,
                                             uint base_min_length,
                                             uint desired_min_length,
                                             uint desired_max_length) const {
    uint min_young_length = desired_min_length - base_min_length;
    const double base_time_ms = 
        predict_base_elapsed_time_ms(pending_cards, rs_length);
    const uint available_free_regions = _free_regions_at_end_of_collection;
    // _reserve_regions 是 244
    const uint base_free_regions =
        available_free_regions > _reserve_regions ? available_free_regions - _reserve_regions : 0;
    G1YoungLengthPredictor p(collector_state()->mark_or_rebuild_in_progress(),
                             base_time_ms,
                             base_free_regions,
                             target_pause_time_ms,
                             this);
    if (p.will_fit(min_young_length)) {
        if (p.will_fit(max_young_length)) {
            min_young_length = max_young_length;
        } else {
        }
    }
    return base_min_length + min_young_length;
}

分析如下：

参数

  • free list num: 2438 - 41 - 65 = 2332
  • safety_factor: 2.2
  • young_length: 121
  • _reserve_regions: 2438 * 10% = 243.8 = 244(ceil)
  • _base_free_regions: 2332 - 244 = 2088
  • free_bytes: (2088 - 121) * 8Mb = 1967 * 8Mb = 15736Mb
  • expected_bytes_to_copy: 1967 / 2.2 * 8Mb = 894 * 8Mb = 7184Mb

条件

  • condition 1: 不好说，因为 _base_free_regions 在每次 GC 后就会把 eden 区清空并加入该列表
  • condition 2: 上一次是 79ms，而最大值是 400ms
  • condition 3: 如果走到了这里，则 _base_free_regions 至少大于 365。_base_free_regions - young_length >= 244

假设 _available_free_regions == 2321，则

  • base_free_regions = 2321 - 244 = 2087
  • _base_free_regions - young_length = 2087 - 121 = 1966 (free_bytes)
  • safety_factor = (100 / 50) * (100 + 10) / 100 = 2.2
  • 需要 copy 的空间 >= 894

min_young_length

class G1YoungLengthPredictor {
    const double _base_time_ms;
    const double _base_free_regions;
    const double _target_pause_time_ms;
    bool will_fit(uint young_length) const {
        // young_length 在出问题的时刻是 80，因为 121-41=80
        // _base_free_regions = available_free_regions - _reserve_regions
        // available_free_regions = 2438 - 41 - 65 = 2332
        // _reserve_regions = 244
        // _base_free_regions = 2332 - 244 = 2088
        // 80 >= 2088，所以是 false
        if (young_length >= _base_free_regions) {
            // end condition 1: not enough space for the young regions
            return false;
        }
        const double accum_surv_rate = _policy->accum_yg_surv_rate_pred((int) young_length - 1);
        const size_t bytes_to_copy = (size_t) (accum_surv_rate * 
                (double) HeapRegion::GrainBytes);
        const double copy_time_ms = _policy->analytics()->
                predict_object_copy_time_ms(bytes_to_copy, _during_cm);
        const double young_other_time_ms = _policy->analytics()->
                predict_young_other_time_ms(young_length);
        const double pause_time_ms = _base_time_ms + copy_time_ms 
                + young_other_time_ms;
        // _target_pause_time_ms == 400
        if (pause_time_ms > _target_pause_time_ms) {
            // end condition 2: prediction is over the target pause time
            return false;
        }
        // 2088 - 80 = 2008
        const size_t free_bytes = (_base_free_regions - young_length) * HeapRegion::GrainBytes;
        // safety_factor = 2.2
        const double safety_factor = (100.0 / G1ConfidencePercent) * (100 + TargetPLABWastePct) / 100.0;
        // 极端假设，所有 eden 区都会复制到 survivor 或者 old, 
        // 那么 bytes_to_copy 最大 region 个数是 121 
        // 即使 121 * 2.2 = 266.2，所以远远达不到 2008 这个水平
        const size_t expected_bytes_to_copy = (size_t) (safety_factor * bytes_to_copy);
        if (expected_bytes_to_copy > free_bytes) {
            // end condition 3: out-of-space
            return false;
        }
        // success!
        return true;
    }
};

max_young_length

class G1YoungLengthPredictor {
    const double _base_time_ms;
    const double _base_free_regions;
    const double _target_pause_time_ms;
    bool will_fit(uint young_length) const {
        // young_length 在出问题之前的时刻是 80，因为 1950-52=1898
        // _base_free_regions = available_free_regions - _reserve_regions
        // available_free_regions = 2438 - 52 - 65 = 2321
        // _reserve_regions = 244
        // _base_free_regions = 2321 - 244 = 2077
        // 1898 >= 2077，所以为假
        if (young_length >= _base_free_regions) {
            // end condition 1: not enough space for the young regions
            return false;
        }
        const double accum_surv_rate = _policy->accum_yg_surv_rate_pred((int) young_length - 1);
        // accum_surv_rate <= 81
        const size_t bytes_to_copy = (size_t) (accum_surv_rate * 
                (double) HeapRegion::GrainBytes);
        const double copy_time_ms = _policy->analytics()->
                predict_object_copy_time_ms(bytes_to_copy, _during_cm);
        const double young_other_time_ms = _policy->analytics()->
                predict_young_other_time_ms(young_length);
        const double pause_time_ms = _base_time_ms + copy_time_ms 
                + young_other_time_ms;
        // _target_pause_time_ms == 400
        if (pause_time_ms > _target_pause_time_ms) {
            // end condition 2: prediction is over the target pause time
            return false;
        }
        // (2077 - 1898) * 8Mb = 179 * 8Mb
        const size_t free_bytes = (_base_free_regions - young_length) * HeapRegion::GrainBytes;
        // safety_factor = 2.2
        const double safety_factor = (100.0 / G1ConfidencePercent) * (100 + TargetPLABWastePct) / 100.0;
        const size_t expected_bytes_to_copy = (size_t) (safety_factor * bytes_to_copy);
        // expected_bytes_to_copy <= 179 * 8Mb / 2.2 = 81 * 8Mb
        if (expected_bytes_to_copy > free_bytes) {
            // end condition 3: out-of-space
            return false;
        }
        // success!
        return true;
    }
};

_reserve_regions 赋值

void G1Policy::record_new_heap_size(uint new_number_of_regions) {
    double reserve_regions_d = (double) new_number_of_regions * _reserve_factor;
    _reserve_regions = (uint) ceil(reserve_regions_d);
}

_free_regions_at_end_of_collection 赋值

void G1Policy::record_collection_pause_end(double pause_time_ms, bool concurrent_operation_is_full_mark) {
    _free_regions_at_end_of_collection = _g1h->num_free_regions();
}
class G1CollectedHeap : public CollectedHeap {
    uint num_free_regions() const { return _hrm.num_free_regions(); }
};
class HeapRegionManager: public CHeapObj<mtGC> {
    FreeRegionList _free_list;
    uint num_free_regions() const {
        return _free_list.length();
    }
};

calculate_young_list_target_length 调用

G1Policy::YoungTargetLengths G1Policy::young_list_target_lengths(size_t rs_length) const {
    YoungTargetLengths result;
    young_list_target_length =
                        calculate_young_list_target_length(rs_length,
                                                           base_min_length,
                                                           desired_min_length,
                                                           desired_max_length);
    result.first = young_list_target_length;
    return result;
}
uint G1Policy::update_young_list_target_length(size_t rs_length) {
    YoungTargetLengths young_lengths = young_list_target_lengths(rs_length);
    _young_list_target_length = young_lengths.first;
    return young_lengths.second;
}
uint G1Policy::update_young_list_max_and_target_length(size_t rs_length) {
    uint unbounded_target_length = update_young_list_target_length(rs_length);
}
uint G1Policy::update_young_list_max_and_target_length() {
    return update_young_list_max_and_target_length(_analytics->predict_rs_length());
}
void G1Policy::record_collection_pause_end(double pause_time_ms, bool concurrent_operation_is_full_mark) {
    size_t last_unrestrained_young_length = update_young_list_max_and_target_length();
}
void G1CollectedHeap::do_collection_pause_at_safepoint_helper(double target_pause_time_ms) {
    policy()->record_collection_pause_end(pause_time_ms, concurrent_operation_is_full_mark);
}

解决思路

日志配置

-Xlog:gc*=info,gc+phases=debug:file=/usr/local/tomcat/logs/gc.log:time,tags,level

效果如下

[2022-02-16T10:19:46.747+0800][info ][gc,start      ] GC(47) Pause Young (Normal) (G1 Evacuation Pause)
[2022-02-16T10:19:46.747+0800][info ][gc,task       ] GC(47) Using 13 workers of 13 for evacuation
[2022-02-16T10:19:46.811+0800][info ][gc,phases     ] GC(47)   Pre Evacuate Collection Set: 0.0ms
[2022-02-16T10:19:46.811+0800][debug][gc,phases     ] GC(47)     Prepare TLABs: 0.0ms
[2022-02-16T10:19:46.811+0800][debug][gc,phases     ] GC(47)     Choose Collection Set: 0.0ms
[2022-02-16T10:19:46.811+0800][debug][gc,phases     ] GC(47)     Humongous Register: 0.0ms
[2022-02-16T10:19:46.811+0800][info ][gc,phases     ] GC(47)   Evacuate Collection Set: 63.3ms
[2022-02-16T10:19:46.811+0800][debug][gc,phases     ] GC(47)     Ext Root Scanning (ms):   Min:  0.0, Avg:  0.0, Max:  0.1, Diff:  0.1, Sum:  0.4, Workers: 13
[2022-02-16T10:19:46.811+0800][debug][gc,phases     ] GC(47)     Update RS (ms):           Min:  0.6, Avg:  0.8, Max:  1.1, Diff:  0.5, Sum:  9.9, Workers: 13
[2022-02-16T10:19:46.811+0800][debug][gc,phases     ] GC(47)       Processed Buffers:        Min: 1, Avg:  1.7, Max: 4, Diff: 3, Sum: 22, Workers: 13
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)       Scanned Cards:            Min: 193, Avg: 255.5, Max: 364, Diff: 171, Sum: 3321, Workers: 13
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)       Skipped Cards:            Min: 0, Avg:  0.0, Max: 0, Diff: 0, Sum: 0, Workers: 13
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     Scan RS (ms):             Min:  0.0, Avg:  0.0, Max:  0.0, Diff:  0.0, Sum:  0.1, Workers: 13
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)       Scanned Cards:            Min: 0, Avg:  0.2, Max: 2, Diff: 2, Sum: 2, Workers: 13
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)       Claimed Cards:            Min: 0, Avg:  0.2, Max: 2, Diff: 2, Sum: 2, Workers: 13
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)       Skipped Cards:            Min: 0, Avg:  0.0, Max: 0, Diff: 0, Sum: 0, Workers: 13
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     Code Root Scanning (ms):  Min:  0.0, Avg:  0.0, Max:  0.0, Diff:  0.0, Sum:  0.0, Workers: 13
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     AOT Root Scanning (ms):   skipped
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     Object Copy (ms):         Min: 62.1, Avg: 62.4, Max: 62.5, Diff:  0.4, Sum: 811.4, Workers: 13
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     Termination (ms):         Min:  0.0, Avg:  0.0, Max:  0.0, Diff:  0.0, Sum:  0.3, Workers: 13
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)       Termination Attempts:     Min: 45, Avg: 59.4, Max: 71, Diff: 26, Sum: 772, Workers: 13
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     GC Worker Other (ms):     Min:  0.0, Avg:  0.0, Max:  0.0, Diff:  0.0, Sum:  0.1, Workers: 13
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     GC Worker Total (ms):     Min: 63.2, Avg: 63.2, Max: 63.3, Diff:  0.0, Sum: 822.2, Workers: 13
[2022-02-16T10:19:46.812+0800][info ][gc,phases     ] GC(47)   Post Evacuate Collection Set: 0.6ms
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     Code Roots Fixup: 0.0ms
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     Clear Card Table: 0.1ms
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     Reference Processing: 0.0ms
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     Weak Processing: 0.2ms
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     Merge Per-Thread State: 0.0ms
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     Code Roots Purge: 0.0ms
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     Redirty Cards: 0.1ms
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     DerivedPointerTable Update: 0.0ms
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     Free Collection Set: 0.1ms
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     Humongous Reclaim: 0.0ms
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     Start New Collection Set: 0.0ms
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     Resize TLABs: 0.0ms
[2022-02-16T10:19:46.812+0800][debug][gc,phases     ] GC(47)     Expand Heap After Collection: 0.0ms
[2022-02-16T10:19:46.812+0800][info ][gc,phases     ] GC(47)   Other: 0.1ms
[2022-02-16T10:19:46.812+0800][info ][gc,heap       ] GC(47) Eden regions: 125->0(101)
[2022-02-16T10:19:46.812+0800][info ][gc,heap       ] GC(47) Survivor regions: 4->17(17)
[2022-02-16T10:19:46.812+0800][info ][gc,heap       ] GC(47) Old regions: 114->162
[2022-02-16T10:19:46.812+0800][info ][gc,heap       ] GC(47) Humongous regions: 95->57
[2022-02-16T10:19:46.812+0800][info ][gc,metaspace  ] GC(47) Metaspace: 39200K->39200K(552960K)
[2022-02-16T10:19:46.812+0800][info ][gc            ] GC(47) Pause Young (Normal) (G1 Evacuation Pause) 1352M->941M(2048M) 64.412ms
[2022-02-16T10:19:46.812+0800][info ][gc,cpu        ] GC(47) User=0.81s Sys=0.00s Real=0.07s

线程模型

CMS GC 的线程模型

G1 GC 的线程模型

G1 GC 相关

G1 垃圾收集器简介

G1 Young GC 启动条件解析

G1 GC 启动并发标记的解析

G1 Mixed GC 启动条件解析

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