[DISK] Add shard option to disk

dmlc-core

@@ -1 +1 @@
-Subproject commit ad2ddde8b6624abf3007a71b2923c3925530cc81
+Subproject commit 257b09a0ba18625a9fcf3b9471a9b1c35a767b7b

src/common/common.h

@@ -0,0 +1,31 @@
+/*!
+ * Copyright 2015 by Contributors
+ * \file common.h
+ * \brief Common utilities
+ */
+#ifndef XGBOOST_COMMON_COMMON_H_
+#define XGBOOST_COMMON_COMMON_H_
+
+#include <vector>
+#include <string>
+#include <sstream>
+
+namespace xgboost {
+namespace common {
+/*!
+ * \brief Split a string by delimiter
+ * \param s String to be splitted.
+ * \param delim The delimiter.
+ */
+inline std::vector<std::string> Split(const std::string& s, char delim) {
+  std::string item;
+  std::istringstream is(s);
+  std::vector<std::string> ret;
+  while (std::getline(is, item, delim)) {
+    ret.push_back(item);
+  }
+  return ret;
+}
+}  // namespace common
+}  // namespace xgboost
+#endif  // XGBOOST_COMMON_COMMON_H_

src/data/sparse_batch_page.h

@@ -16,6 +16,12 @@
 #include <cstring>
 #include <string>
 #include <utility>
+#include <memory>
+
+#if DMLC_ENABLE_STD_THREAD
+#include <dmlc/concurrency.h>
+#include <thread>
+#endif
 
 namespace xgboost {
 namespace data {
@@ -26,6 +32,8 @@ class SparsePage {
  public:
   /*! \brief Format of the sparse page. */
   class Format;
+  /*! \brief Writer to write the sparse page to files. */
+  class Writer;
   /*! \brief minimum index of all index, used as hint for compression. */
   bst_uint min_index;
   /*! \brief offset of the segments */
@@ -171,6 +179,53 @@ class SparsePage::Format {
   static std::pair<std::string, std::string> DecideFormat(const std::string& cache_prefix);
 };
 
+#if DMLC_ENABLE_STD_THREAD
+/*!
+ * \brief A threaded writer to write sparse batch page to sharded files.
+ */
+class SparsePage::Writer {
+ public:
+  /*!
+   * \brief constructor
+   * \param name_shards name of shard files.
+   * \param format_shards format of each shard.
+   * \param extra_buffer_capacity Extra buffer capacity before block.
+   */
+  explicit Writer(
+      const std::vector<std::string>& name_shards,
+      const std::vector<std::string>& format_shards,
+      size_t extra_buffer_capacity);
+  /*! \brief destructor, will close the files automatically */
+  ~Writer();
+  /*!
+   * \brief Push a write job to the writer.
+   * This function won't block,
+   * writing is done by another thread inside writer.
+   * \param page The page to be wriiten
+   */
+  void PushWrite(std::unique_ptr<SparsePage>&& page);
+  /*!
+   * \brief Allocate a page to store results.
+   *  This function can block when the writer is too slow and buffer pages
+   *  have not yet been recycled.
+   * \param out_page Used to store the allocated pages.
+   */
+  void Alloc(std::unique_ptr<SparsePage>* out_page);
+
+ private:
+  /*! \brief number of allocated pages */
+  size_t num_free_buffer_;
+  /*! \brief clock_pointer */
+  size_t clock_ptr_;
+  /*! \brief writer threads */
+  std::vector<std::unique_ptr<std::thread> > workers_;
+  /*! \brief recycler queue */
+  dmlc::ConcurrentBlockingQueue<std::unique_ptr<SparsePage> > qrecycle_;
+  /*! \brief worker threads */
+  std::vector<dmlc::ConcurrentBlockingQueue<std::unique_ptr<SparsePage> > > qworkers_;
+};
+#endif  // DMLC_ENABLE_STD_THREAD
+
 /*!
  * \brief Registry entry for sparse page format.
  */

src/data/sparse_page_dmatrix.cc

@@ -12,34 +12,42 @@
 #if DMLC_ENABLE_STD_THREAD
 #include "./sparse_page_dmatrix.h"
 #include "../common/random.h"
+#include "../common/common.h"
 #include "../common/group_data.h"
 
 namespace xgboost {
 namespace data {
 
-SparsePageDMatrix::ColPageIter::ColPageIter(std::unique_ptr<dmlc::SeekStream>&& fi)
+SparsePageDMatrix::ColPageIter::ColPageIter(
-    : fi_(std::move(fi)), page_(nullptr) {
+    std::vector<std::unique_ptr<dmlc::SeekStream> >&& files)
+    : page_(nullptr), clock_ptr_(0), files_(std::move(files)) {
   load_all_ = false;
+  formats_.resize(files_.size());
+  prefetchers_.resize(files_.size());
 
+  for (size_t i = 0; i < files_.size(); ++i) {
+    dmlc::SeekStream* fi = files_[i].get();
     std::string format;
-  CHECK(fi_->Read(&format)) << "Invalid page format";
+    CHECK(fi->Read(&format)) << "Invalid page format";
-  format_.reset(SparsePage::Format::Create(format));
+    formats_[i].reset(SparsePage::Format::Create(format));
-  size_t fbegin = fi_->Tell();
+    SparsePage::Format* fmt = formats_[i].get();
-
+    size_t fbegin = fi->Tell();
-  prefetcher_.Init([this](SparsePage** dptr) {
+    prefetchers_[i].reset(new dmlc::ThreadedIter<SparsePage>(4));
+    prefetchers_[i]->Init([this, fi, fmt] (SparsePage** dptr) {
         if (*dptr == nullptr) {
           *dptr = new SparsePage();
         }
         if (load_all_) {
-        return format_->Read(*dptr, fi_.get());
+          return fmt->Read(*dptr, fi);
         } else {
-        return format_->Read(*dptr, fi_.get(), index_set_);
+          return fmt->Read(*dptr, fi, index_set_);
         }
-    }, [this, fbegin] () {
+      },  [this, fi, fbegin] () {
-      fi_->Seek(fbegin);
+        fi->Seek(fbegin);
         index_set_ = set_index_set_;
         load_all_ = set_load_all_;
       });
+  }
 }
 
 SparsePageDMatrix::ColPageIter::~ColPageIter() {
@@ -47,10 +55,12 @@ SparsePageDMatrix::ColPageIter::~ColPageIter() {
 }
 
 bool SparsePageDMatrix::ColPageIter::Next() {
+  // doing clock rotation over shards.
   if (page_ != nullptr) {
-    prefetcher_.Recycle(&page_);
+    size_t n = prefetchers_.size();
+    prefetchers_[(clock_ptr_ + n - 1) % n]->Recycle(&page_);
   }
-  if (prefetcher_.Next(&page_)) {
+  if (prefetchers_[clock_ptr_]->Next(&page_)) {
     out_.col_index = dmlc::BeginPtr(index_set_);
     col_data_.resize(page_->offset.size() - 1, SparseBatch::Inst(nullptr, 0));
     for (size_t i = 0; i < col_data_.size(); ++i) {
@@ -60,18 +70,26 @@ bool SparsePageDMatrix::ColPageIter::Next() {
     }
     out_.col_data = dmlc::BeginPtr(col_data_);
     out_.size = col_data_.size();
+    // advance clock
+    clock_ptr_ = (clock_ptr_ + 1) % prefetchers_.size();
     return true;
   } else {
     return false;
   }
 }
 
+void SparsePageDMatrix::ColPageIter::BeforeFirst() {
+  clock_ptr_ = 0;
+  for (auto& p : prefetchers_) {
+    p->BeforeFirst();
+  }
+}
+
 void SparsePageDMatrix::ColPageIter::Init(const std::vector<bst_uint>& index_set,
                                           bool load_all) {
   set_index_set_ = index_set;
   set_load_all_ = load_all;
   std::sort(set_index_set_.begin(), set_index_set_.end());
-
   this->BeforeFirst();
 }
 
@@ -103,8 +121,9 @@ ColIterator(const std::vector<bst_uint>& fset) {
 
 bool SparsePageDMatrix::TryInitColData() {
   // load meta data.
+  std::vector<std::string> cache_shards = common::Split(cache_info_, ':');
   {
-    std::string col_meta_name = cache_prefix_ + ".col.meta";
+    std::string col_meta_name = cache_shards[0] + ".col.meta";
     std::unique_ptr<dmlc::Stream> fmeta(
         dmlc::Stream::Create(col_meta_name.c_str(), "r", true));
     if (fmeta.get() == nullptr) return false;
@@ -112,13 +131,15 @@ bool SparsePageDMatrix::TryInitColData() {
     CHECK(fmeta->Read(&col_size_)) << "invalid col.meta file";
   }
   // load real data
-  {
+  std::vector<std::unique_ptr<dmlc::SeekStream> > files;
-    std::string col_data_name = cache_prefix_ + ".col.page";
+  for (const std::string& prefix : cache_shards) {
+    std::string col_data_name = prefix + ".col.page";
     std::unique_ptr<dmlc::SeekStream> fdata(
         dmlc::SeekStream::CreateForRead(col_data_name.c_str(), true));
     if (fdata.get() == nullptr) return false;
-    col_iter_.reset(new ColPageIter(std::move(fdata)));
+    files.push_back(std::move(fdata));
   }
+  col_iter_.reset(new ColPageIter(std::move(files)));
   return true;
 }
 
@@ -135,24 +156,17 @@ void SparsePageDMatrix::InitColAccess(const std::vector<bool>& enabled,
   buffered_rowset_.clear();
   col_size_.resize(info.num_col);
   std::fill(col_size_.begin(), col_size_.end(), 0);
-  // make the sparse page.
-  dmlc::ThreadedIter<SparsePage> cmaker;
-  SparsePage tmp;
-  size_t batch_ptr = 0, batch_top = 0;
   dmlc::DataIter<RowBatch>* iter = this->RowIterator();
   std::bernoulli_distribution coin_flip(pkeep);
-
+  size_t batch_ptr = 0, batch_top = 0;
+  SparsePage tmp;
   auto& rnd = common::GlobalRandom();
 
   // function to create the page.
   auto make_col_batch = [&] (
       const SparsePage& prow,
       const bst_uint* ridx,
-      SparsePage **dptr) {
+      SparsePage *pcol) {
-    if (*dptr == nullptr) {
-      *dptr = new SparsePage();
-    }
-    SparsePage* pcol = *dptr;
     pcol->Clear();
     pcol->min_index = ridx[0];
     int nthread;
@@ -199,7 +213,7 @@ void SparsePageDMatrix::InitColAccess(const std::vector<bool>& enabled,
     }
   };
 
-  auto make_next_col = [&] (SparsePage** dptr) {
+  auto make_next_col = [&] (SparsePage* dptr) {
     tmp.Clear();
     size_t btop = buffered_rowset_.size();
 
@@ -236,41 +250,44 @@ void SparsePageDMatrix::InitColAccess(const std::vector<bool>& enabled,
     }
   };
 
-  cmaker.Init(make_next_col, []() {});
+  std::vector<std::string> cache_shards = common::Split(cache_info_, ':');
-
+  std::vector<std::string> name_shards, format_shards;
-  std::string col_data_name = cache_prefix_ + ".col.page";
+  for (const std::string& prefix : cache_shards) {
-  std::unique_ptr<dmlc::Stream> fo(dmlc::Stream::Create(col_data_name.c_str(), "w"));
+    name_shards.push_back(prefix + ".col.page");
-  // find format.
+    format_shards.push_back(SparsePage::Format::DecideFormat(prefix).second);
-  std::string name_format = SparsePage::Format::DecideFormat(cache_prefix_).second;
+  }
-  fo->Write(name_format);
+  SparsePage::Writer writer(name_shards, format_shards, 6);
-  std::unique_ptr<SparsePage::Format> format(SparsePage::Format::Create(name_format));
+  std::unique_ptr<SparsePage> page;
+  writer.Alloc(&page); page->Clear();
 
   double tstart = dmlc::GetTime();
   size_t bytes_write = 0;
   // print every 4 sec.
   const double kStep = 4.0;
   size_t tick_expected = kStep;
-  SparsePage* pcol = nullptr;
 
-  while (cmaker.Next(&pcol)) {
+  while (make_next_col(page.get())) {
-    for (size_t i = 0; i < pcol->Size(); ++i) {
+    for (size_t i = 0; i < page->Size(); ++i) {
-      col_size_[i] += pcol->offset[i + 1] - pcol->offset[i];
+      col_size_[i] += page->offset[i + 1] - page->offset[i];
     }
-    format->Write(*pcol, fo.get());
+
-    size_t spage = pcol->MemCostBytes();
+    bytes_write += page->MemCostBytes();
-    bytes_write += spage;
+    writer.PushWrite(std::move(page));
+    writer.Alloc(&page);
+    page->Clear();
+
     double tdiff = dmlc::GetTime() - tstart;
     if (tdiff >= tick_expected) {
-      LOG(CONSOLE) << "Writing to " << col_data_name
+      LOG(CONSOLE) << "Writing col.page file to " << cache_info_
                    << " in " << ((bytes_write >> 20UL) / tdiff) << " MB/s, "
                    << (bytes_write >> 20UL) << " MB writen";
       tick_expected += kStep;
     }
-    cmaker.Recycle(&pcol);
   }
   // save meta data
-  std::string col_meta_name = cache_prefix_ + ".col.meta";
+  std::string col_meta_name = cache_shards[0] + ".col.meta";
-  fo.reset(dmlc::Stream::Create(col_meta_name.c_str(), "w"));
+  std::unique_ptr<dmlc::Stream> fo(
+    dmlc::Stream::Create(col_meta_name.c_str(), "w"));
   fo->Write(buffered_rowset_);
   fo->Write(col_size_);
   fo.reset(nullptr);

src/data/sparse_page_dmatrix.h

@@ -14,6 +14,7 @@
 #include <algorithm>
 #include <string>
 #include "./sparse_batch_page.h"
+#include "../common/common.h"
 
 namespace xgboost {
 namespace data {
@@ -21,9 +22,9 @@ namespace data {
 class SparsePageDMatrix : public DMatrix {
  public:
   explicit SparsePageDMatrix(std::unique_ptr<DataSource>&& source,
-                             const std::string& cache_prefix)
+                             const std::string& cache_info)
-      : source_(std::move(source)),
+      : source_(std::move(source)), cache_info_(cache_info) {
-        cache_prefix_(cache_prefix) {}
+  }
 
   MetaInfo& info() override {
     return source_->info;
@@ -77,11 +78,9 @@ class SparsePageDMatrix : public DMatrix {
   // declare the column batch iter.
   class ColPageIter : public dmlc::DataIter<ColBatch> {
    public:
-    explicit ColPageIter(std::unique_ptr<dmlc::SeekStream>&& fi);
+    explicit ColPageIter(std::vector<std::unique_ptr<dmlc::SeekStream> >&& files);
     virtual ~ColPageIter();
-    void BeforeFirst() override {
+    void BeforeFirst() override;
-      prefetcher_.BeforeFirst();
-    }
     const ColBatch &Value() const override {
       return out_;
     }
@@ -90,20 +89,22 @@ class SparsePageDMatrix : public DMatrix {
     void Init(const std::vector<bst_uint>& index_set, bool load_all);
 
    private:
-    // data file pointer.
-    std::unique_ptr<dmlc::SeekStream> fi_;
     // the temp page.
     SparsePage* page_;
+    // internal clock ptr.
+    size_t clock_ptr_;
+    // data file pointer.
+    std::vector<std::unique_ptr<dmlc::SeekStream> > files_;
     // page format.
-    std::unique_ptr<SparsePage::Format> format_;
+    std::vector<std::unique_ptr<SparsePage::Format> > formats_;
+    /*! \brief internal prefetcher. */
+    std::vector<std::unique_ptr<dmlc::ThreadedIter<SparsePage> > > prefetchers_;
     // The index set to be loaded.
     std::vector<bst_uint> index_set_;
     // The index set by the outsiders
     std::vector<bst_uint> set_index_set_;
     // whether to load data dataset.
     bool set_load_all_, load_all_;
-    // data prefetcher.
-    dmlc::ThreadedIter<SparsePage> prefetcher_;
     // temporal space for batch
     ColBatch out_;
     // the pointer data.
@@ -117,7 +118,7 @@ class SparsePageDMatrix : public DMatrix {
   // source data pointer.
   std::unique_ptr<DataSource> source_;
   // the cache prefix
-  std::string cache_prefix_;
+  std::string cache_info_;
   /*! \brief list of row index that are buffered */
   std::vector<bst_uint> buffered_rowset_;
   // count for column data

src/data/sparse_page_source.cc

@@ -9,35 +9,45 @@
 
 #if DMLC_ENABLE_STD_THREAD
 #include "./sparse_page_source.h"
+#include "../common/common.h"
 
 namespace xgboost {
 namespace data {
 
-SparsePageSource::SparsePageSource(const std::string& cache_prefix)
+SparsePageSource::SparsePageSource(const std::string& cache_info)
-    : base_rowid_(0), page_(nullptr) {
+    : base_rowid_(0), page_(nullptr), clock_ptr_(0) {
-  // read in the info files.
+  // read in the info files
+  std::vector<std::string> cache_shards = common::Split(cache_info, ':');
+  CHECK_NE(cache_shards.size(), 0);
   {
-    std::string name_info = cache_prefix;
+    std::string name_info = cache_shards[0];
     std::unique_ptr<dmlc::Stream> finfo(dmlc::Stream::Create(name_info.c_str(), "r"));
     int tmagic;
     CHECK_EQ(finfo->Read(&tmagic, sizeof(tmagic)), sizeof(tmagic));
     this->info.LoadBinary(finfo.get());
   }
+  files_.resize(cache_shards.size());
+  formats_.resize(cache_shards.size());
+  prefetchers_.resize(cache_shards.size());
+
   // read in the cache files.
-  std::string name_row = cache_prefix + ".row.page";
+  for (size_t i = 0; i < cache_shards.size(); ++i) {
-  fi_.reset(dmlc::SeekStream::CreateForRead(name_row.c_str()));
+    std::string name_row = cache_shards[i] + ".row.page";
-
+    files_[i].reset(dmlc::SeekStream::CreateForRead(name_row.c_str()));
+    dmlc::SeekStream* fi = files_[i].get();
     std::string format;
-  CHECK(fi_->Read(&format)) << "Invalid page format";
+    CHECK(fi->Read(&format)) << "Invalid page format";
-  format_.reset(SparsePage::Format::Create(format));
+    formats_[i].reset(SparsePage::Format::Create(format));
-  size_t fbegin = fi_->Tell();
+    SparsePage::Format* fmt = formats_[i].get();
-
+    size_t fbegin = fi->Tell();
-  prefetcher_.Init([this] (SparsePage** dptr) {
+    prefetchers_[i].reset(new dmlc::ThreadedIter<SparsePage>(4));
+    prefetchers_[i]->Init([fi, fmt] (SparsePage** dptr) {
         if (*dptr == nullptr) {
           *dptr = new SparsePage();
         }
-      return format_->Read(*dptr, fi_.get());
+        return fmt->Read(*dptr, fi);
-    }, [this, fbegin] () { fi_->Seek(fbegin); });
+      }, [fi, fbegin] () { fi->Seek(fbegin); });
+  }
 }
 
 SparsePageSource::~SparsePageSource() {
@@ -45,12 +55,16 @@ SparsePageSource::~SparsePageSource() {
 }
 
 bool SparsePageSource::Next() {
+  // doing clock rotation over shards.
   if (page_ != nullptr) {
-    prefetcher_.Recycle(&page_);
+    size_t n = prefetchers_.size();
+    prefetchers_[(clock_ptr_ + n - 1) % n]->Recycle(&page_);
   }
-  if (prefetcher_.Next(&page_)) {
+  if (prefetchers_[clock_ptr_]->Next(&page_)) {
     batch_ = page_->GetRowBatch(base_rowid_);
     base_rowid_ += batch_.size;
+    // advance clock
+    clock_ptr_ = (clock_ptr_ + 1) % prefetchers_.size();
     return true;
   } else {
     return false;
@@ -59,33 +73,48 @@ bool SparsePageSource::Next() {
 
 void SparsePageSource::BeforeFirst() {
   base_rowid_ = 0;
-  prefetcher_.BeforeFirst();
+  clock_ptr_ = 0;
+  for (auto& p : prefetchers_) {
+    p->BeforeFirst();
+  }
 }
 
 const RowBatch& SparsePageSource::Value() const {
   return batch_;
 }
 
-bool SparsePageSource::CacheExist(const std::string& cache_prefix) {
+bool SparsePageSource::CacheExist(const std::string& cache_info) {
-  std::string name_info = cache_prefix;
+  std::vector<std::string> cache_shards = common::Split(cache_info, ':');
-  std::string name_row = cache_prefix + ".row.page";
+  CHECK_NE(cache_shards.size(), 0);
+  {
+    std::string name_info = cache_shards[0];
     std::unique_ptr<dmlc::Stream> finfo(dmlc::Stream::Create(name_info.c_str(), "r", true));
+    if (finfo.get() == nullptr) return false;
+  }
+  for (const std::string& prefix : cache_shards) {
+    std::string name_row = prefix + ".row.page";
     std::unique_ptr<dmlc::Stream> frow(dmlc::Stream::Create(name_row.c_str(), "r", true));
-  return finfo.get() != nullptr && frow.get() != nullptr;
+    if (frow.get() == nullptr) return false;
+  }
+  return true;
 }
 
 void SparsePageSource::Create(dmlc::Parser<uint32_t>* src,
-                              const std::string& cache_prefix) {
+                              const std::string& cache_info) {
+  std::vector<std::string> cache_shards = common::Split(cache_info, ':');
+  CHECK_NE(cache_shards.size(), 0);
   // read in the info files.
-  std::string name_info = cache_prefix;
+  std::string name_info = cache_shards[0];
-  std::string name_row = cache_prefix + ".row.page";
+  std::vector<std::string> name_shards, format_shards;
-  std::unique_ptr<dmlc::Stream> fo(dmlc::Stream::Create(name_row.c_str(), "w"));
+  for (const std::string& prefix : cache_shards) {
-  std::string name_format = SparsePage::Format::DecideFormat(cache_prefix).first;
+    name_shards.push_back(prefix + ".row.page");
-  fo->Write(name_format);
+    format_shards.push_back(SparsePage::Format::DecideFormat(prefix).first);
-  std::unique_ptr<SparsePage::Format> format(SparsePage::Format::Create(name_format));
+  }
+  SparsePage::Writer writer(name_shards, format_shards, 6);
+  std::unique_ptr<SparsePage> page;
+  writer.Alloc(&page); page->Clear();
 
   MetaInfo info;
-  SparsePage page;
   size_t bytes_write = 0;
   double tstart = dmlc::GetTime();
   // print every 4 sec.
@@ -107,14 +136,16 @@ void SparsePageSource::Create(dmlc::Parser<uint32_t>* src,
       info.num_col = std::max(info.num_col,
                               static_cast<uint64_t>(index + 1));
     }
-    page.Push(batch);
+    page->Push(batch);
-    if (page.MemCostBytes() >= kPageSize) {
+    if (page->MemCostBytes() >= kPageSize) {
-      bytes_write += page.MemCostBytes();
+      bytes_write += page->MemCostBytes();
-      format->Write(page, fo.get());
+      writer.PushWrite(std::move(page));
-      page.Clear();
+      writer.Alloc(&page);
+      page->Clear();
+
       double tdiff = dmlc::GetTime() - tstart;
       if (tdiff >= tick_expected) {
-        LOG(CONSOLE) << "Writing to " << name_row << " in "
+        LOG(CONSOLE) << "Writing row.page to " << cache_info << " in "
                      << ((bytes_write >> 20UL) / tdiff) << " MB/s, "
                      << (bytes_write >> 20UL) << " written";
         tick_expected += kStep;
@@ -122,57 +153,62 @@ void SparsePageSource::Create(dmlc::Parser<uint32_t>* src,
     }
   }
 
-  if (page.data.size() != 0) {
+  if (page->data.size() != 0) {
-    format->Write(page, fo.get());
+    writer.PushWrite(std::move(page));
   }
 
-  fo.reset(dmlc::Stream::Create(name_info.c_str(), "w"));
+  std::unique_ptr<dmlc::Stream> fo(
+      dmlc::Stream::Create(name_info.c_str(), "w"));
   int tmagic = kMagic;
   fo->Write(&tmagic, sizeof(tmagic));
   info.SaveBinary(fo.get());
-
+  LOG(CONSOLE) << "SparsePageSource: Finished writing to " << name_info;
-  LOG(CONSOLE) << "SparsePageSource: Finished writing to " << cache_prefix;
 }
 
 void SparsePageSource::Create(DMatrix* src,
-                              const std::string& cache_prefix) {
+                              const std::string& cache_info) {
+  std::vector<std::string> cache_shards = common::Split(cache_info, ':');
+  CHECK_NE(cache_shards.size(), 0);
   // read in the info files.
-  std::string name_info = cache_prefix;
+  std::string name_info = cache_shards[0];
-  std::string name_row = cache_prefix + ".row.page";
+  std::vector<std::string> name_shards, format_shards;
-  std::unique_ptr<dmlc::Stream> fo(dmlc::Stream::Create(name_row.c_str(), "w"));
+  for (const std::string& prefix : cache_shards) {
-  // find format.
+    name_shards.push_back(prefix + ".row.page");
-  std::string name_format = SparsePage::Format::DecideFormat(cache_prefix).first;
+    format_shards.push_back(SparsePage::Format::DecideFormat(prefix).first);
-  fo->Write(name_format);
+  }
-  std::unique_ptr<SparsePage::Format> format(SparsePage::Format::Create(name_format));
+  SparsePage::Writer writer(name_shards, format_shards, 6);
+  std::unique_ptr<SparsePage> page;
+  writer.Alloc(&page); page->Clear();
 
-  SparsePage page;
+  MetaInfo info;
   size_t bytes_write = 0;
   double tstart = dmlc::GetTime();
   dmlc::DataIter<RowBatch>* iter = src->RowIterator();
 
   while (iter->Next()) {
-    page.Push(iter->Value());
+    page->Push(iter->Value());
-    if (page.MemCostBytes() >= kPageSize) {
+    if (page->MemCostBytes() >= kPageSize) {
-      bytes_write += page.MemCostBytes();
+      bytes_write += page->MemCostBytes();
-      format->Write(page, fo.get());
+      writer.PushWrite(std::move(page));
-      page.Clear();
+      writer.Alloc(&page);
+      page->Clear();
       double tdiff = dmlc::GetTime() - tstart;
-      LOG(CONSOLE) << "Writing to " << name_row << " in "
+      LOG(CONSOLE) << "Writing to " << cache_info << " in "
                    << ((bytes_write >> 20UL) / tdiff) << " MB/s, "
                    << (bytes_write >> 20UL) << " written";
     }
   }
 
-  if (page.data.size() != 0) {
+  if (page->data.size() != 0) {
-    format->Write(page, fo.get());
+    writer.PushWrite(std::move(page));
   }
 
-  fo.reset(dmlc::Stream::Create(name_info.c_str(), "w"));
+  std::unique_ptr<dmlc::Stream> fo(
+      dmlc::Stream::Create(name_info.c_str(), "w"));
   int tmagic = kMagic;
   fo->Write(&tmagic, sizeof(tmagic));
-  src->info().SaveBinary(fo.get());
+  info.SaveBinary(fo.get());
-
+  LOG(CONSOLE) << "SparsePageSource: Finished writing to " << name_info;
-  LOG(CONSOLE) << "SparsePageSource: Finished writing to " << cache_prefix;
 }
 
 }  // namespace data

src/data/sparse_page_source.h

@@ -71,14 +71,14 @@ class SparsePageSource : public DataSource {
   RowBatch batch_;
   /*! \brief page currently on hold. */
   SparsePage *page_;
-  /*! \brief The cache predix of the dataset. */
+  /*! \brief internal clock ptr */
-  std::string cache_prefix_;
+  size_t clock_ptr_;
   /*! \brief file pointer to the row blob file. */
-  std::unique_ptr<dmlc::SeekStream> fi_;
+  std::vector<std::unique_ptr<dmlc::SeekStream> > files_;
   /*! \brief Sparse page format file. */
-  std::unique_ptr<SparsePage::Format> format_;
+  std::vector<std::unique_ptr<SparsePage::Format> > formats_;
   /*! \brief internal prefetcher. */
-  dmlc::ThreadedIter<SparsePage> prefetcher_;
+  std::vector<std::unique_ptr<dmlc::ThreadedIter<SparsePage> > > prefetchers_;
 };
 }  // namespace data
 }  // namespace xgboost

src/data/sparse_page_writer.cc

@@ -0,0 +1,72 @@
+/*!
+ * Copyright (c) 2015 by Contributors
+ * \file sparse_batch_writer.cc
+ * \param Writer class sparse page.
+ */
+#include <xgboost/base.h>
+#include <xgboost/logging.h>
+#include "./sparse_batch_page.h"
+
+#if DMLC_ENABLE_STD_THREAD
+namespace xgboost {
+namespace data {
+
+SparsePage::Writer::Writer(
+    const std::vector<std::string>& name_shards,
+    const std::vector<std::string>& format_shards,
+    size_t extra_buffer_capacity)
+    : num_free_buffer_(extra_buffer_capacity + name_shards.size()),
+      clock_ptr_(0),
+      workers_(name_shards.size()),
+      qworkers_(name_shards.size()) {
+  CHECK_EQ(name_shards.size(), format_shards.size());
+  // start writer threads
+  for (size_t i = 0; i < name_shards.size(); ++i) {
+    std::string name_shard = name_shards[i];
+    std::string format_shard = format_shards[i];
+    auto* wqueue = &qworkers_[i];
+    workers_[i].reset(new std::thread(
+        [this, name_shard, format_shard, wqueue] () {
+          std::unique_ptr<dmlc::Stream> fo(
+              dmlc::Stream::Create(name_shard.c_str(), "w"));
+          std::unique_ptr<SparsePage::Format> fmt(
+              SparsePage::Format::Create(format_shard));
+          fo->Write(format_shard);
+          std::unique_ptr<SparsePage> page;
+          while (wqueue->Pop(&page)) {
+            fmt->Write(*page, fo.get());
+            qrecycle_.Push(std::move(page));
+          }
+          fo.reset(nullptr);
+          LOG(CONSOLE) << "SparsePage::Writer Finished writing to " << name_shard;
+        }));
+  }
+}
+
+SparsePage::Writer::~Writer() {
+  for (auto& queue : qworkers_) {
+    queue.SignalForKill();
+  }
+  for (auto& thread : workers_) {
+    thread->join();
+  }
+}
+
+void SparsePage::Writer::PushWrite(std::unique_ptr<SparsePage>&& page) {
+  qworkers_[clock_ptr_].Push(std::move(page));
+  clock_ptr_ = (clock_ptr_ + 1) % workers_.size();
+}
+
+void SparsePage::Writer::Alloc(std::unique_ptr<SparsePage>* out_page) {
+  CHECK(out_page->get() == nullptr);
+  if (num_free_buffer_ != 0) {
+    out_page->reset(new SparsePage());
+    --num_free_buffer_;
+  } else {
+    CHECK(qrecycle_.Pop(out_page));
+  }
+}
+}  // namespace data
+}  // namespace xgboost
+
+#endif  // DMLC_ENABLE_STD_THREAD