Entity Framework Core 中的内存管理

Question

我试图请求大量数据，然后将其解析为报告。问题是我请求的数据有 2700 万行记录，每行有 6 个连接，当通过实体框架加载时会使用所有服务器 RAM。我已经实现了一个分页系统来将处理缓冲成更小的块，就像你对 IO 操作所做的那样。

我请求 10,000 条记录，将它们写入文件流（写入磁盘），我正尝试从内存中清除 10,000 条记录，因为它们不再需要。

我在垃圾收集数据库上下文时遇到了麻烦。我已经尝试处理对象，将引用清空，然后在下一批 10,000 条记录上创建新上下文。这似乎不起作用。 （这是 ef core 上的一位开发人员推荐的：https://github.com/aspnet/EntityFramework/issues/5473）

我看到的唯一其他选择是使用原始 SQL 查询来实现我想要的。我正在尝试构建系统来处理任何请求大小，唯一的可变因素是生成报告所需的时间。我可以用 EF 上下文做些什么来摆脱加载的实体吗？

 private void ProcessReport(ZipArchive zip, int page, int pageSize)
        {
            using (var context = new DBContext(_contextOptions))
            {
                var batch = GetDataFromIndex(page, pageSize, context).ToArray();
                if (!batch.Any())
                {
                    return;
                }

                var file = zip.CreateEntry("file_" + page + ".csv");
                using (var entryStream = file.Open())
                using (var streamWriter = new StreamWriter(entryStream))
                {
                    foreach (var reading in batch)
                    {
                        try
                        {
                            streamWriter.WriteLine("write data from record here.")
                        }
                        catch (Exception e)
                        {
                            //handle error
                        }
                    }
                }
                batch = null;
            }
            ProcessReport(zip, page + 1, pageSize);
        }

private IEnumerable<Reading> GetDataFromIndex(int page, int pageSize, DBContext context)
        {

            var batches = (from rb in context.Reading.AsNoTracking()
                //Some joins
                select rb)
                .Skip((page - 1) * pageSize)
                .Take(pageSize);

                return batches
                    .Includes(x => x.Something)

        }

Answer 1

除了您的内存管理问题之外，您将很难为此使用分页。在服务器上运行分页查询会变得很昂贵。你不需要翻页。只需迭代查询结果（即不要调用 ToList() 或 ToArray()）。

另外，当分页时，您必须为查询添加排序，否则 SQL 可能会返回重叠的行，或者有间隙。请参阅 SQL Server，例如：https://docs.microsoft.com/en-us/sql/t-sql/queries/select-order-by-clause-transact-sql EF Core 不强制执行此操作，因为某些提供程序可能会保证分页查询始终以相同的顺序读取行。

以下是 EF Core（.NET Core 上的 1.1）在不增加内存使用量的情况下通过庞大的结果集的示例：

using Microsoft.EntityFrameworkCore;
using System.Linq;
using System;
using System.ComponentModel.DataAnnotations.Schema;

namespace efCoreTest
{
    [Table("SomeEntity")]
    class SomeEntity
    {

        public int Id { get; set; }
        public string Name { get; set; }
        public string Description { get; set; }

        public DateTime CreatedOn { get; set; }
        public int A { get; set; }
        public int B { get; set; }
        public int C { get; set; }
        public int D { get; set; }

        virtual public Address Address { get; set; }
        public int AddressId { get; set; }

    }

    [Table("Address")]
    class Address
    {
        [DatabaseGenerated(DatabaseGeneratedOption.None)]
        public int Id { get; set; }
        public string Line1 { get; set; }
        public string Line2 { get; set; }
        public string Line3 { get; set; }

    }
    class Db : DbContext
    {
        public DbSet<SomeEntity> SomeEntities { get; set; }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlServer("Server=.;Database=efCoreTest;Integrated Security=true");
        }

    }
    class Program
    {
        static void Main(string[] args)
        {
            using (var db = new Db())
            {
                db.Database.EnsureDeleted();
                db.Database.EnsureCreated();

                db.Database.ExecuteSqlCommand("alter database EfCoreTest set recovery simple;");

                var LoadAddressesSql = @"

with N as
(
   select top (10) cast(row_number() over (order by (select null)) as int) i
   from sys.objects o, sys.columns c, sys.columns c2
)
insert into Address(Id, Line1, Line2, Line3)
select i Id, 'AddressLine1' Line1,'AddressLine2' Line2,'AddressLine3' Line3
from N;
";

                var LoadEntitySql = @"

with N as
(
   select top (1000000) cast(row_number() over (order by (select null)) as int) i
   from sys.objects o, sys.columns c, sys.columns c2
)
insert into SomeEntity (Name, Description, CreatedOn, A,B,C,D, AddressId)
select  concat('EntityName',i) Name,
        concat('Entity Description which is really rather long for Entity whose ID happens to be ',i) Description,
        getdate() CreatedOn,
        i A, i B, i C, i D, 1+i%10 AddressId
from N

";
                Console.WriteLine("Generating Data ...");
                db.Database.ExecuteSqlCommand(LoadAddressesSql);
                Console.WriteLine("Loaded Addresses");

                for (int i = 0; i < 10; i++)
                {
                    var rows = db.Database.ExecuteSqlCommand(LoadEntitySql);
                    Console.WriteLine($"Loaded Entity Batch {rows} rows");
                }


                Console.WriteLine("Finished Generating Data");

                var results = db.SomeEntities.AsNoTracking().Include(e => e.Address).AsEnumerable();

                int batchSize = 10 * 1000;
                int ix = 0;
                foreach (var r in results)
                {
                    ix++;

                    if (ix % batchSize == 0)
                    {
                        Console.WriteLine($"Read Entity {ix} with name {r.Name}.  Current Memory: {GC.GetTotalMemory(false) / 1024}kb GC's Gen0:{GC.CollectionCount(0)} Gen1:{GC.CollectionCount(1)} Gen2:{GC.CollectionCount(2)}");

                    }

                }

                Console.WriteLine($"Done.  Current Memory: {GC.GetTotalMemory(false)/1024}kb");

                Console.ReadKey();
            }
        }
    }
}

输出

Generating Data ...
Loaded Addresses
Loaded Entity Batch 1000000 rows
Loaded Entity Batch 1000000 rows
. . .
Loaded Entity Batch 1000000 rows
Finished Generating Data
Read Entity 10000 with name EntityName10000.  Current Memory: 2854kb GC's Gen0:7 Gen1:1 Gen2:0
Read Entity 20000 with name EntityName20000.  Current Memory: 4158kb GC's Gen0:14 Gen1:1 Gen2:0
Read Entity 30000 with name EntityName30000.  Current Memory: 2446kb GC's Gen0:22 Gen1:1 Gen2:0
. . .
Read Entity 9990000 with name EntityName990000.  Current Memory: 2595kb GC's Gen0:7429 Gen1:9 Gen2:1
Read Entity 10000000 with name EntityName1000000.  Current Memory: 3908kb GC's Gen0:7436 Gen1:9 Gen2:1
Done.  Current Memory: 3916kb

注意，EF Core 中内存消耗过多的另一个常见原因是“Mixed client/server evaluation”查询。有关更多信息以及如何禁用自动客户端查询评估，请参阅文档。

Answer 2

这是由于 MARS（多个活动结果集被禁用）造成的。

https://github.com/aspnet/EntityFrameworkCore/issues/9367